It's not a Responsorial Psalm, but a substitute for the verse before the Gospel during Lent; i.e, to replace the Alleluia.

I was ignoring this question, having totally forgotten the source (none of the standard ones have it), and having (to my shame) virtually no notes on the compiling of PBC. By sheer serendipity (though, in fact, he was prompted by your post), I received an email from the culprit who, more or less, insisted we include this bit of liturgical arcana in PBC 2nd ed., and after another search of my bookcase, I finally found the source, which is:

Liber cantualis, Solesmes 1978, p. 20.

Where they got it is anybody's guess; maybe an old Pontificale? maybe Bugnini's hope chest? It still strikes me as borderline spurious (Tracts on the cheap, or something). Does anybody in the Catholic universe actually use this thing?

Thanks Richard. This chant is found in the Cantus Selecti, being the whole of Ps. 50(51) with the response, but I was curious about the provenance of its use as a "Psalmus Responsorius" before the Gospel, since it doesn't follow the format or rubrics of the Responsorial Psalms before the Gospel ("Psalmus Responsorius II") for Lent in the Graduale Simplex, which always have several verses, usually about ten, with the rubric that at least five are sung, since the version of Ps. 50(51) in PBC2 only sets four verses.

I've considered doing it at weekday Masses when there is only one reading before the Gospel, since the Simplex allows for only one chant between the First Reading and the Gospel at weekday Mass, but have always decided against it since people are so accustomed to: Reading, spoken Psalm, sung Gospel Acclamation, Gospel. And since most people follow the Liturgy of the Word in their Missals, not using the Lectionary Psalms throws them off.