looks to be the belgian proof for muzzleloading guns at standard pressures....the overstamp of "23" is obscuring the "G" in the oval....this mark is also seen commonly with a crown atop the oval - which is the extra-pressure version of the proof mark.

i'd say the gun has some belgian connection - barrel sourced there?

best regards,

tom

"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland