All the early 2nd generation Colt trademarked Walkers and Dragoons took #12's just like the originals. This is likely one of those earlier guns.
I have a friend with one of the early Colt 2nd Generation Walkers that I gave a tin of #12's to for collectability. This is a true hartfod horsey logo gun in the hartford horsey logo box. He won't get any more from me though, as I now have three bricks of #12's and I'm a hoarder.
Here's a thread where Gatofeo supports this also:
http://www.go2gbo.com/forums/index.php?topic=9703.0