8. Sequentia, si casus fert, cantatur post ultimum Alleluia alternatim a cantoribus et a choro, vel a duabus partibus chori, omisso Amen in fine. Si non cantatur Alleluia cum suo versu, omittitur Sequentia.

Well, Roman Missal 3 is later legislation than OCM, and could reasonably be interpreted to override the latter in case of conflict.

The 1975 GIRM 40 was not specific, but the Lectionary seems to have had the Sequence sung before the Alleluia. But the 2000 GIRM had it after the Alleluia. The 2002 GIRM returns it to before the Alleluia.

Sequentia was the name given to the jubilus or musical prolongation of the last vowel of the word alleluia. The jubilus is divided into small sections, and to these parts separately as well as to the whole melody the name sequentia could be applied. The custom gradually came into being of adding words or a Prosa to the music of the jubilus. At first, perhaps in the eighth century, a text was added to some of the sections, the last vowel of such texts being, in some places, always the vowel a to which the next wordless section could be sung. Later on a text was added to the whole melody and so began what is now generally called a Sequence or, less generally, a Prose. Its full name would properly be Sequentia cum prosa.
Just as the sung liturgical texts of the Mass are mostly prose, not poetry, so the earliest Proses were, as their name indicates, unmetrical, their structure being dictated by the length and shape of the melody. The first use of such texts is connected with the name of Notker Balbulus, the Stammerer, who was born about 840 and died as a monk of St Gall in Switzerland in 912. But whether such a connection is rightly asserted and, if rightly, what compositions are to be ascribed to him, are matters of great dispute. As time went on, the words were not attached to an existing melody but words and music were composed together. This made for rhythmical structure in prose and, later still, in poetry until the Sequence reached its glory in the compositions of Adam of St Victor (c£ the introduction to Corpus Christi and the notes on its hymns), the Lauda Sion, 74, and the Dies irae, 154. The Victimae, which is in rhythmical prose, belongs to the transitional period of the history of the Sequence.
A Sequence is distinguished, musically, from the alleluia and the alleluia verse by being a syllabic chant, or practically syllabic; and from a hymn by having a new melody for different sections, for instance for each pair of verses or half-verses.