Ho ho. This is one of my pet topics. Stercky's references are all to the various decrees of the Sacred Congregation of Rites or to the Caeremoniale Episcoporum itself. p. 172 of volume 1, Manuel de liturgie, for the Francophones out there. Stercky mentions simply playing the organ (one way or the other) but this means by itself, except where he says that you may not accompany at all.

The prohibitions apply to Advent, to Lent, and to the Ember Days (of September); they have the same exceptions.

The use of solo organ on the Sundays where rose vestments may be worn was limited to Mass and to Vespers (both, by the 20th century), but not at Lauds.

Solo organ is permitted in general when dalmatics are worn, even if the vestments are violet, including in the weeks before Ash Wednesday, on the ferial days following Gaudete Sunday, and on vigils of the season, namely that of Christmas. It is prohibited when folded chasubles are worn. However, by special exception, the organ is not used by itself on vigils of the saints until after the Mass which itself follows None and precedes I Vespers (although this reference is merely to Martinucci and may not be strictly speak of a requirement).

It is permitted when the liturgical function is not of the season, including at Benediction. It is basically expected to be used on feasts which were of obligation and which were holy days in the civil sense as well. Stercky says that this includes all other feasts. I don't know if some communities or chapters omitted solo organ or how, particularly in France, they toned down their solo playing on ferial days per annum or on simple feasts. I do know that while one could essentially always accompany chant, Solesmes abandoned the organ for Lent; Dom Mocquereau comments on this in one of his letters.

Stercky also says that the Gloria may be accompanied on Holy Thursday, or one may play the introduction and fanfare before singing the rest without accompaniment.

The organ may never be used to accompany the responsories of Matins, though it could be used to accompany the psalms on Spy Wednesday. I find this regrettable, as much as I like accompanied psalmody.

Historically, not even polyphony was allowed at the Office of the Dead, including the Mass, before Leo XIII finally changed the rubric, and then the organ was used to accompany the office. However, Matins in general was rarely sung in full, but the organ was typically used. Stercky further claims that it is not customary to use it at the other Hours, except for Terce before pontifical Mass, but it seems that it was customary at Compline by Potiron's day. One uses the organ at solemn Lauds only, essentially in conjunction with Matins when those are sung in full.

Frankly, one problem with the 1958 instruction is that it is poorly drafted and does not admit of a true exception for March 19 and March 25 (or February 24/25 when Ash Wednesday falls early enough for Saint Matthias to fall in Lent), only for extraordinary solemnity.

There are other rules about how the organ may supply verses or play at the elevation and at Benediction.

Would anyone know what the rules were for using the organ at Mass during Lent prior to 1958?

The rules are to be found in book 1, chapter 28 of the Caeremoniale Episcoporum. Generally, the prohibition against organ playing is stricter under the 1958 instruction, but the regulations are exactly the same for Sundays: the organ may be played only to accompany singing and is otherwise silent except on the fourth Sunday. The organ should be completely silent between the Glorias of Holy Thursday and the Easter Vigil; DMS says explicitly that organ accompaniment is not permitted even at private devotions during the Triduum. In the Caeremoniale, organ solos are expressly allowed on the feasts of St. Matthias, St. Thomas Aquinas, St. Gregory the Great, St. Joseph, the Annunciation, and others feasts, whereas DMS only mentions holy days of obligation, holidays (except Sundays), patronal and titular feasts, and the occasion of some extraordinary solemnity. As for ferial days, the CE only says that it is fitting or appropriate (convenit) to observe the same rule as at Masses for the dead, whereas it is required by DMS. Wuest (Matters Liturgical) O'Connell (Sacred Music and Liturgy) was of the opinion that the prohibitions of DMS were intended to apply even at non-seasonal Masses, for instance, a votive Mass with Gloria celebrated in white vestments.

As to ferial days: do you mean those following a green Sunday?

No, the topic at hand is organ accompaniment during Lent.

I find this interesting because our pastor, who typically is very traditional and respectful of Church rules, allows ( requests?) some limited organ playing during Lent, based on French practices. It's also for the Ordinary Form, not the EF.

Rules on when the organ is to be played/not to be played are generally ignored or selectively enforced. So, basically, I'd say do what makes sense for you.

That seems to be the case in our parish.

Well, in France (as in other places, like the Republic of Venice) depending on the reign and the vagaries of relations with Rome, liturgical rules were not always in effective sync with Rome...

Yes. Another reason to have a preparatory period.

maybe, but it's poorly drafted, and the attitude of the time was that reforms only allowed what was specifically permitted. If the plain reading banned or suppressed something, that was to be followed.

Concerts, of course, are not liturgical.

Some background probably lingering in the attic of many people's memory via reading:

https://www.varsity.co.uk/music/15173

And, now rummaging through my attic, the memory that the peak seasons for instrumental concerts as such were Advent and Lent (when operas were not performed):

e.g., https://dickstrawser.blogspot.com/2009/03/concert-of-classical-proportions-focus.html

maybe, but it's poorly drafted, and the attitude of the time was that reforms only allowed what was specifically permitted.

I once heard Fr. Ripperger address this during one of his lectures on the sacred liturgy. He specifically mentioned that the pre-conciliar understanding (as borne out in writings of the time) were staunchly that one could not do a thing unless it was expressly permitted or commanded, whereas after the council, people have taken the attitude that “anything goes” unless it’s specifically forbidden. It is a perfect inversion of the traditionally held understanding (as are so many other things in the wake of the reforms).

I stopped dead in my tracks as I heard this part of the lecture because it clarified so much of what my own experience had been.

Yes, but performed in church? Still not an issue? And what about my work, “Stations of the Cross” and “Mysterium Rosarium”?

Not a problem!

Someone said to me that those who really follow the rules don't use the organ at all during Lent except Laetare Sunday and the Gloria of Holy Thursday,

My understanding of the organ use in Lent rule was that it is tolerated for the purpose of supporting the singing if there wasn’t sufficient skill level to sing a cappella. The rationale being that during Lent we strip away that which brings joy until Easter as we enter into a period of mourning. (ie: no Alleluia, no Gloria, no flowers, no organ (especially postludes), no lace, no singing in solemn tone, covering up statuary, art, etc in Passion Week, no bells after the Gloria on Holy Thursday, but rather replaced with the crotalus, no Mass on Good Friday or Holy Saturday).

I will say this, when the rules are followed, Come Easter when it all returns, it really does bring back a great joy (and not that sappy emotional joy, but true Easter Joy).

Then there are those who argue over whether or not it’s permissible to use a pitch pipe for a starting note on Good Friday...

We also don't use other instruments during the pre-Lent in the old rite, so there are several stages advancing to the full a cappella effect in the Triduum.

Which is hardly an issue in most parishes, but it actually has come up once in my lifetime going to the traditional rite, as odd as I found it myself. No instruments other than organ are allowed after Septuagesima, as Norwalk admirably shows us once again.

Of course, not accompanying things is OK; Solesmes does this to this day, I gather, or at least at the time of Daniel Saulnier's recollections given about twelve years ago at a conference. It's not impossible in a parish, but I notice that unless it's the Asperges, Mass VIII, Credo III, and maybe Credo I, it's hard to get anyone to sing.

And cutting out the organ for accompaniment of the Triuum makes Tenebrae that much more special.

What is the meaning of this?

In the Extraordinary Form there are different preface tones that are sung depending on the liturgical season, particular Mass (ie: a requiem Mass), what level of feast day it is, etc. There’s Solemn tone, which is for regular Sundays, feasts, etc. Ferial tone for Sundays in Lent, Advent, Requiem Masses, vigil Masses, etc. Then there’s More Solemn Tone, which is for Pontifical Masses, Solemnities, etc.

No, the 1962 rules are the same as pre-55, except for the classification of the lowest ranking feasts and certain votive Masses being different but the rules are the same.

Same for the Pater and again, same for both the collect tones follow the same rules as to feast-feria-Requiem.