Barbireau : Kyrie of the mass Virgo parens ChristiCantus
The first ligature means two semibreves and two breves; the second one two semibreves.
Thus, the piece begins with a B S S B sequence which might make you remember a few souvenirs...
Yes indeed, that's an alteration (as can be double-checked by looking at the other voices)! In such a case the two breves remain perfect, thus the result (again in minim units):
6 | 2 4 | 6 | 4 2 | 2 3/2 1/2 1 2 1 1 1/2 1/2 ...