1.) The prominent identifying characteristic on a model 89/90/91 is the magazine. If you've studied enough, you can pick out one on these rifles from 50 feet away based solely on magazine.
2.) Accurate identification using the magazine requires a certain level of knowledge of the model 89/90/91, as well as a certain level of knowledge of the design of other single-stack magazine rifles.
3.) If you have some knowledge, but not enough, you're prone to making mistakes.
4.) The rifle that most resembles the Carcano isn't a Mauser '91, it's a Gew88, which isn't a coincidence, BTW. OK, maybe a Mannlicher M1893.
Sure, but who would take a "glance" at a rifle and make up all that crap, even if he did know the caliber of a Mauser and how many bullets the clip held? Especially for a sworn affidavit pertaining to the assassination of the POTUS! No one would do that. Weitzman was obviously pressured into recanting his sworn testimony and concede that he only glanced at the rifle and guessed it was a Mauser. Sure, his affidavit was a guesstimate. Only a LNer believes that.