I replied on the ShotgunWorld forum. But I will add it here as well. The long "serial number" on the frame makes me think it was in Japan at some point. They used these long numbers stamped on the frame as either a control number or an export number. This would also make sense with there being no makers name and odd proof marks.

There were several Japanese makers coping English shotguns pre WWII. Some had 2 1/2" chambers, most I have seen were 2 3/4" or more with very long forcing cones (for the time) 1" long or more.

Here are a few pictures of my Japanese guns for reference.
In order: Mashiro, Ito Hailand, unsigned (possibly Kasuga), B&K Hamada

Note the long control/export number stamped on the first 3 water tables.

Also note the odd proof marks on the third gun. Looks like they were trying to copy Birmingham proofs, but added a KA. Also didn't get all of the characters in NITRO PROOF.