KingOfKindling
40 Cal
I suspect what you have is a second-generation Colt, similar to this one:
https://www.riverjunction.com/3662
The above gun is said to be made in the 1970s, and based on these serial numbers, that would seem correct, as it would for yours:
http://proofhouse.com/colt/1851navy.htm
Yeah, I think you're dead on the money here. Same finish color on bluing and case, same stamping, same serial # range, and the same trigger guard.
This is a second generation gun not an original. Which makes a lot more sense.