Thanks for the replies. I really appreciate the time and information. A member on CGN came up wth the company I was trying to remember. The name of the company was Centaur. They reloaded surplus cases to milspec standards with 55 grain FMJ. The cartridges were never sold to the US or any other nation's military forces that I know of. On the other hand, they also reloaded pistol ammunition and did sell it to security companies as well as police forces. Some of this stuff ended up on the surplus market and a few shops carried it. The 5.56x45 was supposedly loaded in the same pressure range and milspec, around 57,000psi.

Thanks again Bruce M, I hadn't realised that the powder used in the 54-70 and the 45 Colt was the same powder as that used in cannons of the period. It certainly makes sense though. Especially in the frontier areas.