If the '07 SMLE Bayonet were to fit.. Here are some measurements off of a SMLE MkI* (approx 1906) nose cap/bayonet attachment lug.

The muzzle round diameter .650" (SMLE doesn't use the bbl itself, but a separate lug below it on the nose cap)
The rear lug width .490
Rear ug channel width .240
Distance from the back face of the rear lug to the rear edge of the round muzzle portion 3.90"

The SMLE Bayonet Lug does not have that small cut (locking cut) on one side seen in the photo of the rifle posted.

My guess is not a 98 bayonet as they don't have a ring mount up front, just a U shaped guide.
Perhaps one of the pre-98 Mauser styles does have the ring that would encompass the barrel muzzle and also have the grip locking mechanism.
My knowledge of bayonets is about exhausted in what I've posted.

Perhaps it just a mount for an already mentioned light.
Way out front and on the bottom of the bbl seems like a good spot for it, but again just a guess.

Just some thoughts. Very interesting rifle.