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Welcome to the forum, and congrats on deciding on a nice gift for your BF !
Ditto on getting an expert to physically examine your prospect - It's easy for an uneducated person to get "taken"
FWIW, the "pattern" you showed here (round bbl, fluted/stepped cylinder) certainly looks like a .36cal (same cal as Model 1851) Model 1862 Police Colt (which was available with 5-1/2" bbl) - and the bbl address is correct for that model.
If the case is an original Colt case, in good condition, the gun may be fairly priced.
FWIW only early .44 cal Model 1860 Colt Army's had fluted cylinders (AFAIK).
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