Actually, I think that Colt can letter the signature series.
"REVOLVERS
Agent, Army Special, Anaconda, Bankers Special, Border Patrol (c. 1970), Cobra, Commando, Courier, Deringer (4th Model), Lord & Lady Deringer, Detective Special, Diamondback, Frontier Scout, King Cobra, Marshal, New Frontier .22, Officer's Model, Officer's Model Match, Official Police, Pequano, Pocket Positive, Police Positive, Police Positive Special, Python, Peacemaker .22, Three-fifty-seven Magnum, Trooper,
Signature Series Black Powder Models, Reproduction Black Powder models"
At least they are listed on their site:
http://www.coltsmfg.com/cmci/archive_prices.asp
Jared