The Proof marks equate to the 1855 to 1868 period and if the serial number is 1664 then that would fit in that period. No indication of it ever have been a pin-fire conversion in my opinion. As English guns are hand made and each is unique as a result it would be virtually impossible to smuggle out in pieces. One Colt revolver would be much the same as the other and would be easy to do that with but most certainly not in the case of an English gun especially a Boss. Looks okay to me. Best to contact Boss and quote that number and see where it leads. Lagopus.....