Busnois : Kyrie of the mass O Crux lignumTenor
I'm sure you'll find out the details: a few imperfections on left or right sides.
Taking them in account, you'll see that the first line sums up to fifteen breves (if we include the rests), and the second one to twelve, these two numbers being multiples of three.
As a matter of fact, this agrees with the laying of the rests at the beginning of the piece, which suggest a perfect long!
However, it seems that it's here more a tradition (?) than a real necessity, since the piece doesn't contain any note longer than a breve...