Thanks very much, Mike. If the letter in question is to be trusted, the Colt rifles appear to have been issued to the Scandinavian Guards, a New Orleans volunteer militia company which became Co. A of the Chalmette Regiment. As to the M1855 designation, that is not from the original source, it's a usage that I've encountered in modern works. BTW, do you have a date for the capture of the Baton Rouge arsenal?

Lars