I know you're supposed to only load five, but I could see it being possible that in the 19th century, they would load six, put it on half cock, and not care about safety. Is there any evidence of the Army being taught to only load five, or is it modern common sense?
That's a great question. Have you tried going to the army and asking if the have any training manuals from that period on display? Otherwise small museums and living history groups will often have republished manuals of arms, etc.
Also Colt themselves may have a collection of materials that can help.