Second Vespers for Sunday, Week II in the contemporary Antiphonale Romanum call for the antiphon Nos qui vivimus (CAO 3960). This antiphon does not appear in the 1933 Antiphonarium O.P., but does appear in the 13th century Dominican manuscript tradition in conjunction with the antiphons of Sunday vespers presented after Epiphany used on most Sundays of the year.
Curiously, the two 13th century Dominican Antiphonals I have consulted contain what might be called a “cautionary natural” before the B in “benedicimus”. In other sources for this chant, the B often has a flat (including in the contemporary Antiphonale Romanum), or is pitched starting on G rather than C such that the upper note is F, which is equivalent to B-flat if starting on C (the Antiphonale Synopticum presents various versions transcribed in this manner), or in some cases has a different melodic structure that doesn’t ascend above the A. In one 13th or early 14th century Franciscan source, there is a B without an accidental, which presumably is meant to be B-natural. It would be interesting to get a broader sense of the breadth of versions of this antiphon.
It is interesting to see that the scribes for this Dominican version are deliberately calling attention to what was likely an unusual version of the melody. Notably, the London scribe does not supply a flat for the B in the incipit of the tonus perigrinus, whereas the Rome scribe has included it in the expected place.