Tim:
The "39" mark on the flats is the bore diameter of the rifled barrel. Pre-1887, the British marked bore diameter of all barrels in gauge - 39 gauge is for a rifled .500 barrel. This is not a .577/.450, or at least was not originally. Assuming that the seller is correct and a .577/.450 MH will chamber....that doesn't make sense to me. No way a .577/.450 will chamber in one of the straight cased .500s, which suggests one of the bottlenecked .577 based .500s, such as the .577/.500 No. 2. However, a .577/.450 MH shouldn't chamber in one of those either (base diameter should still be too large).
Diameter of the shot barrel SHOULD be marked. If it's a cape gun, both barrels wouldn't be marked "39". There should be a 12, 16, etc., somewhere on the smooth barrel.
The "1646" on the bottom rib is the trade maker's serial number.
The pictures will help. Good luck.
