What what the caliber means, about its possible age, history, etc. I don't even know what its called to try and date it.
This is an interesting problem.
How is it that you have had this Colt (and the .45) for some time without knowing what it is? Did you inherit them? Are you in the habit of buying a pig in a poke?
Furnish the serial number and somebody will look up the year made.
As to history, if you pay Colt Archive Services $100 they will tell you when and where it was shipped from the factory and what its configuration was when new. (I don't think the two-piece walnut grip panels are stock items.) But there is no way to know where it went after that.