Dear Martin, have you considered the possibility that the
sign you read as u at the end is A) in fact a virāma; or B) a
scribal error for what was meant to be a virāma? Given that it
is a north Indian manuscript, a Telugu-ish ending seems
unlikely as you say. I am sure I have seen "saṃvat" written
(on a copper plate in Nagari) with a virāma that looked very
much like an u. I have no particular recollection of ever
seeing saṃmat instead of saṃvat, but it saṃmat would not be a
strange development.
Daniel