Maybe looking through this list of Mott mower manuals will help determine your model.
Alamo Industrial: Manuals - Current Production Manuals Model numbers are usually by inches of width to help narrow it down. I'd have to guess part numbers would be the same as far as bearings etc. unless upgraded, but you can always check part numbers on bearing retainers, etc. with what you have.
I'd have to guess the bolt loosening is due to sudden pto engagement. Even though you may be engaging it at an idle, some tractors that the independent pto is engaged hydraulically, will slam a pto implement when starting up, instead of one's you can manually feather engagement. If the drum rotates in the opposite direction, than the bolt tightens, it can loosen up, if continually slammed, if not torqued properly.
In the one particular operators manual I looked at, it gave torque specs for bolts, by size. If you have a torque wrench, you might try determining bolt size, then torque to specs. If it still does it, I'd be tempted to get some Blue Loctite threadlocker, and apply it to the bolt, then torque it down, and see if that cures the problem.
