Also check the quality of the inletting for the sling ring bar - should be crisp and even.
John's point about a splice under the barrel band is a good one, and could answer my concern about the shaping of the front of the wood. If it's an added piece, the band spring inletting would have been hand done and might show some crudeness.
I like John's last sentence as well. If the work was done a long time ago - even if not right - it would show a uniformity of finish/wear. Fifty years ago, many collectors were a LOT more tolerant of little oddities on their pieces - the good, extensively researched, books did not come out until the early 80's. If the auction was due to the collector having passed, some of his stuff was probably acquired during the 100 year period of (relative) ignorance.
Then, on the other hand, HOW, exactly, did SA determine the form the carbine was going to take? No actual "1866 Carbines" are known, and way less than ten 1868s are recorded. Nothing was produced in "quantity" until the 341 Model 1870s, of which I own a specimen. This gun was not made from "junk" because the (rare) early bar and correct early sling ring appear to be right, albeit for an 1870. The fabricator had access to at least SOME "good stuff". And, did the 1870 Carbine stock simply copy something that had been mocked-up earlier?
Looking at the pics again, that is a strange thumb latch. I do not know if an 1868 (or later) latch could even be fitted into an 1866 block. There are some 'odd' profiles to the wood around the rear of the action (almost wrote "receiver") as well, including the lack of a proper cut for the latch arm. Of course, if it was "the first attempt" they might have gotten it wrong. There is an odd mix of features; that's for sure, since any modern faker would have known things that the maker of this appear not to have. It's a puzzle.
Still, the odds are overwhelmingly in favor of a made-up piece, but the TINY possibility that you have found an SA sample needs to be eliminated before a definite statement can be made. If it were me I'd surely take it to the show at Timonium.