The gun is a WWII vintage British Enfield Revolver, officially Pistol, Revolver, No. 2 Mk 1*. It was the British standard handgun of WWII.
The caliber is (or was originally), .38 Smith & Wesson, an older and shorter round than the .38 Special.
The barrel was originally 5 inch, but after the guns were sold as surplus, importers had some converted to short barrels for sale in the U.S.
The early guns (Mark 1) had a hammer spur and could be cocked for single action firing, but the spur tended to snag when tankers to whom the gun was issued were trying to get out of the tank (often in a great hurry) so the Mk 1* (pronounced "Mark One star") was issued without the hammer spur and could be fired only double action.
Sometimes called a Webley because of similarity to a revolver made by that company, it is not a Webley and was never made by Webley. From what is left of the marking on the right side, it appears yours was made by the Royal Small Arms Factory at Enfield Lock. Other makers were Albion Motors in England and Howard Auto Cultivators in Australia.
The markings on the front right of the frame are British commercial proof marks, put on when the gun was tested after it was sold by the government and converted. The V38xx is the serial number; the FTRT 51 means the gun was overhauled by the British military in 1951. (FTR means Factory Thorough Repair; the second "T" indicates the repair facility, but I can't identify it.)
I am not sure of the cylinder marking, but I think I can safely state that the gun was never used by or issued by the FBI. It may be an inspection mark.
Jim
