"To begin with, the rifle only fooled one law enforcement officer:"
I guess I'm not explaining myself clear enough.
The TSBD Carcano wasn't mistaken for a model 1891 Argentine 7.65 mauser by any of the LE officers present.
That Mauser's build quality and distintive markings compared to the Carcano make the idea one was mistaken for the other improbable.
We already figured out your first assertion. The second requires that you have someone who not only knows the difference between the two rifles, and is in a position to be able to clearly see said markings. In the right light, you can see engraving on a rifle from an arms length away. Don't count on reading it much further away than that. And that's in good light.
"Nothing implies Weitzman possessed any particularly useful knowledge regarding identifying firearms"
https://www.maryferrell.org/showDoc.html?docId=9926&relPageId=34&search=lane_and%20weisberg%20and%20mauser
"In addition to Weitzman having a familiarity of rifles because of his sporting good operation and his own interest, Weitzman was an engineer, a graduate engineer."
Weizman had an engineering degree, which in itself has nothing to do with whether Weitzman knew anything at all about firearms. And, by the time he encountered the rifle in the TSBD, he'd spent the bulk of his adult life in the rag trade. His sole foray into "the sporting goods business" was the year --if that-- he spent winding down a small-chain "discount operation."
"Boone had only been a Deputy for about a year, and supposedly holds the record as the youngest deputy that Decker ever hired.
I doubt he was a firearms expert"
Boone didn't need to be a firearms expert to tell the difference between a Carcano and a Argentine Mauser. This is Dallas, Texas, a gun culture, and as pointed out by Officer Marion Baker in his WC testimony.
SENATOR COOPER - Have you fired other types of rifles other than the one you used?
Mr. BAKER - Yes, sir; the first one I had was a 30-30 Marlin lever type.
SENATOR COOPER - Have you ever seen the rifle that is alleged to have belonged to Lee Oswald?
Mr. BAKER - I saw it, a photograph of it, in the newspaper.
SENATOR COOPER - Do you know what kind of rifle it is?
Mr. BAKER - Not offhand. I heard it was some foreign make gun. Most of the boys down there at the police
department have had dealings with foreign type guns, rifles, you know of this kind, and a lot of them sell them,
and a lot of them rework them, you know, make them into deer rifles.
I'm well aware of "Dallas, Texas gun culture." In fact, I'm part of it. And as a part of it, I can tell you that there are guys out there who own a half-dozen model 98's who would be mystified by a model 91 if they saw it on someone's wall. The '91's are mechanically different from the later Mausers in a number of ways. Because of that, they aren't as sought after, and are somewhat obscure any more.
Anyway, in order to tell the difference between a Mauser '91 and a Carcano, you have to know what they look like. Where did Boone say that he knew what they look like? For that matter, how many of those "Dallas gun culture" cops saw the rifle and said it was definitely a 7.65 Mauser?