I'd guess, and it's just a guess of course, that any factory piece that was done for a sitting President of the US would have some pretty good paperwork to back it up. Even if the provenance is not with the actual gun, the guns existance would be well known.

Colt's engraved SAA that was in the process of being finished up for JFK when he was murdered was left just as that, in the unfinished state but it was near completion.
It's well known factory piece, well documented and pictured. Same as the other factory pieces presented to the other Presidents.
I kind of doubt S&W would let a photo-op like a presentation to the POTUS go by the wayside.
If done and not presented because of the events in Dallas, the gun would still have factory record as the 19 above.

Many engraved pieces are and were done aftermarket in homage to persons or events in our history.
There are also many that have been done with the idea to fake a connection betw the recipient and the mgf'r of the piece.
Let's make another Czar of Russia Parker shotgun. He never received that one either.
Or how many FE Special AH Fox shotguns with the same ser# can show up at the same large G/Show,,all with a factory letterof course,,answ..3

A common come-back for an good engraver when offered a 'rare' presentation piece is,,,'How many would you like?'

I do hope the revolver turns out to be something of factory work and presentation to JFK.
But without absolute ironclad provenance (I love that word!), I'd look at it as another nicely done revolver with an interesting story that is probably worth digging in to.
What do the collectors always say?,,Buy the gun not the story....

Just my .02