First, there is no Colt Model 1874 revolver.
The first Colt DA revolvers were the Model 1877 "Rainmaker", the Model 1877 "Lightning, the Model 1877 "Thunderer", and the Model 1878 DA.
Only the Model 1877 Thunderer and the Model 1878 DA were made in the .41 Colt Long.
The patent dates referred to the original patents applied for before actual production began in 1877.
You can do a Google search for pictures of how to identify whether you have a Model 1877 or a Model 1878.
Here's two sources of shipping date: NOTE All Colt firearms started with serial number 1 back then. You have to identify whether you have a Model 1877 or 1878 to get a valid date.
Colt Firearms Company look-up:
http://www.coltsmfg.com/CustomerServices/SerialNumberLookup.aspx
Proof House data base:
http://proofhouse.com/colt/
More info:
You can buy an Archive letter from Colt that will list what Colt knows about the gun. This would include the configuration it left the factory in (Caliber, barrel length, any custom order features) when it was shipped, and who it was shipped to.
Values on guns are based on the actual amount of ORIGINAL factory finish, whether it has original grips and is in the configuration it left the factory in, and whether it's in working order.
The standard industry reference is The Blue Book of Gun Values which lists value on percentage of original finish.
Values listed for an 1877 Thunderer are as follows, with the caution that condition is everything and is best judged by an expert. Your 90% may be an experts 95% or it could be an 80%.
Also, values on Colt firearms are changing fast so values may be higher then listed.
60%--$975
70%--$1,125
80%--$1,675
90%--$2,100
95%--$2,250
If the gun is a Model 1878:
60%--$1,750
70%--$2,500
80%--$3,000
90%--$3,750
95%--#4,750