Go here: http://www.marstar.ca/AssemblyColt25AP.htm
Best left to a gunsmith
You can get a letter from Colt. It will cost from 75 on up depeding on specifics. Check their web site.
Best to seek help from a qualified gunsmith.
If you are looking for a manual, go to the link below and scroll down to Colt .25 hammerless for a magazine try: edssportshop.com
check out: http://stevespages.com/page7b.htm