I'm tardy here but thank you all for your thoughts. I haven't got a letter on this one and I understand one is not available. I bought the gun several years ago from a dealer friend who had purchased it in an Auction of a well known Colt Collector's collection. The gun had passed through the hands of 3 knowledgable and experienced people and it passed my own, less knowledgable and experienced, sniff test so the lack of a letter is of less concern.
The Auction description states the gun is original with a Military barrel, except for a slightly reduced front sight, and that it is factory nickel. I think that opinion is largely due to the absence of any buffing which is supported by the lack of any nickel in the stamps, except the barrel. In the experience of many, even a hint of buffing will open the stampings to allow the easy flow of a nickeling the stampings.
The barrel is an expected decision here because is was a military barrel left over from Colts Military contracts. It was likely already blued. So I think it is logical that Colt stamped it with the new civilian serial number and refinished it in Nickel when it nickeled the rest of the gun.
By its Serial No. the barrel is a Military barrel. While not itself condemned, it likely came from a condemned Cavalry. The last four digits of its Military SN are "2758." Thus, its Military Serial No. is either 2757 or 12757. Both would be Ainsworth stamped and his "A" stamp is above the "P" in the 8th photo ( sorry for the poor quality).
Because of the deterioration of the italic barrel address, the consensious is that its Military SN is 12757.
I was drawn to this SAA because its such a good example of the lengths Colt went to use all available parts. Note also how the barrel was altered to accept the Type II Ejector Housing. When the first runs of the Military Contacts ended around SN 20,000, Colt rushed to fill the orders backlog from the civilian market. The use of left over parts form the Military orders in the guns having SN's in the low 20,000's is not uncommon.
That's one frustration of Colt collecting. There are always other possibile explanations.
My opinion...original and restamped by Colt. That nickel looks a 140 years old. (IMO)