Hi suprslik.......wecome to TFF!
To give you an approximate value of your 1851 Navy, we'll need to know the serial # and the % of the original finish left.
As for it's authenticity, that can only be determined by a "hands-on" examination by a Colt expert.
Are you sure the barrel is 5"? Should be 7 1/2".
Also, that address should be NEW-YORK CITY or NEW-YORK U.S. AMERICA. Which is it?