Howdy
The number on the loading gate is an assembly number. It will not be the same as the Serial Number.
The Serial Number of record on a Colt Single Action Army is on the under side of the frame. The upper number in this photo. The serial number on First Generation Colts (made before 1941) should also appear on the trigger guard as shown in this photo. You will notice I have obliterated the last two digits of the serial numbers.
The serial number on 1st Gen Colts should also appear on the under side of the butt, like this:
If the SNs on the trigger guard and/or butt do not match the SN on the frame, it means the gun did not leave the factory with those parts, which will lower the value.
From what I can read of the Serial Number, it appears this Colt was made in the second year of production, 1874. It also appears to be in spectacular condition. Note that spectacular is my own term, it is not an official collector's designation. I am a little bit suspicious of the bright blue on the trigger guard screws, and the one piece wooden grip is in terrific condition for a gun that old.
This Colt may or may not have been refinished at some time, the colors of the Case Hardened frame are almost too good to be true. Other than a minor amount of corrosion on the barrel, the blue looks to be in terrific condition. There is also a very minimal turn line on the cylinder, meaning either that it has hardly been shot at all, or that somebody who knew how to take care of an old Colt owned it for a long time.
It is impossible to state an actual value of this revolver without actual first hand examination by an expert. No amount of photos will do, it is too easy to doctor photos.
You potentially have a valuable piece there, perhaps worth $4000 - $5000 on the collector's market. Again, that is only a ball park estimate based on what I can see on the photos.
I found myself in a similar situation last year when I was asked to take a look at a collection of guns from a deceased WWII veteran. I discovered there was what was potentially a very valuable Colt 1911 Semi-Automatc pistol in the collection.
Rather than refer the owner to a local dealer, which was my first intention, I called up a local auction house. The problem with a dealer is they may or may not be honest, and they are in business to make a profit. At best with a dealer you can expect to get about 50% of the value of a gun, and that is if they are honest. An auction house operates differently, they will drive the price up as far as possible, and they only take a fixed percentage of the sale. So the market will determine the value, and you will have signed a contract in advance stipulating the auction house's cut. In addition a reputable auction house will have an expert on hand who can evaluate the gun, determine if the finish is original or not, and place a market value on the gun. In my case the auction house owner was very interested, he came down and inspected the gun, determined it was legitimate, and took the entire collection. The owner of the collection made out very well.
Where are you located?
It is best if there is an auction house within a days drive so the expert can inspect the gun himself.
I would definitely stay away from on line auctions, only a brick and mortar auction house.
As for books, good luck. The most authoritative book on Colt Single Action Army revolvers is 'A Study of the Colt Single Action Army Revolver', by John Kopec. It is 600 pages long, and if you can find a copy online it costs about $150. And it will not tell you much about current auction values.
The auction house I went to is the Amoskeag Auction Company in Manchester New Hampshire. You can browse through their past auctions on line for a description of the guns and what prices were paid. Note, the price listed is not the 'hammer price' it includes the 15% premium that went to the auction house.
http://www.amoskeagauction.com/index.php