I've been browsing through the posts over on the Colt forum, and have a couple of additional data points for Willy and some others.
Colt SAA #8599 was part of a shipment of 50 to Schuyler, Hartley, and Graham in NYC, and shipped July 8, 1874. This example, at least, was in 45 Colt. Incidentally, this gun is martially-marked--probably either a reject for some reason, or an overrun--and was purportedly owned by WF Cody.
This means that not only were they available on the market by then (SH&G was a wholesaler, shipping onesy-twosy to retailers across the country), but 45 Colt ammo was on the market as well.