First thing, as to the 1891 date in the ad from my original post. The 1891 date is there because the ad appeared in an 1891 issue of Shooting and Fishing
magazine. One would think that this is supposed to represent the years Colt was in business, i.e. from 1856 to 1891.
As we all know, Colt started much earlier than that with his Paterson rifles and revolvers. Now, I can perhaps understand Colt not wanting to remind people of his earlier bankruptcy at Paterson, thus he did not want to use the now commonly accepted "start date" of 1836. But why use 1856 as opposed to say 1847 or 1848, after the success of his Walker, Dragoon, and Baby Dragoon revolvers?
My GUESS would be that since Colt's new state of the art factory was completed in 1856 at Hartford, that this MAY have something to do with using the date of 1856. There may also have been some reorganization or restructuring of the company in 1856, thus requiring that date to be used. I dunno, just guesses
To show that the 1856 date is not a typo, pictured below are a couple of more ads, one from 1889 and the other 1893. Colt ran these ads in Shooting and Fishing almost weekly
over these years (and perhaps longer, as I stopped checking at 1893). Thus, it seems unlikely that a typo would go for that long of a period.