Well, some people refer to it as the 1866 Cadet - since it IS built on the 2nd Allin action. Only 424 were made, and they led a rough life after leaving the service - few survive.
I did not own one when I wrote my .58/.50 book which was published in 2006. Now, I do, but have not yet photographed it in detail, beyond the one shot (picture #10 in album #1) on my webpage www.picturetrail.com/sa4570af. Difference in butt-stock length is quite evident, and it is also shorter than the M1866 Short Rifle, with which it is sometines confused. Adding to the difficulty in ID'ing cadets of that period is the fact that several of the surplus dealers cut 3-band rifles down (in a couple of different configurations) and called them cadets. Interesting, but not the rare SA original.
The 1867 is the miniaturized one, where the butt and length of pull are reduced, and there are a number of special parts which do not interchange with full-size TDs. Even the band-springs are slightly shorter than normal. The lockplate is still bevelled, but is thinner, and is ONLY dated "1867". All subsequent cadets were much more interchangeable - just 3" shorter between the bands and with a slimmer stock.