Mysterious Mauser Carbine with"OT" serial number

[765mm]

From what you have descibed it sounds like a Spanish model 1916. They are the only mauser that I know of that has a serial number starting with OT, followed by 4 to 6 digits (OT123456).

Spain rebuilt many of their earlier rifles around WWI and up to WWII to a short rifle version, reusing what they had instead of buying new rifles.

If it is a Mod.1916 it would have started out life as a Model 1893 rifle or maybe a 1895 carbine.
The rifles were stripped down and many had the crests removed as well as the makers address and model number on the left side of the receiver, while some had new crests applied, like the Guardia Civil model.
Most, but not all, also had gas relief holes drilled in the side of the chamber as well as having the left receiver rail cut out as a thumb relief to make it easier to load the rifle with a stripper clip.

One way to tell if it is a Mod.1916 or not is by the bolt face. Does it have a 98 style bolt or something earlier?
Since the Spanish Mod. 1916 started out as a Mod. 1893 it will have a flat bottom bolt. The 93 is the only Mauser that used this bolt.
Just look at the bolt face. If the bottom is flat, it is a 93.... If it is rounded, it's not a 93.

For more info on the Spanish models 1893 and 1916 go to Spanmauhome it is one of the best sites for info and identification of these models.

Hope this helps,
Walter