I think Bsa sold them a batch of underlevers in the 30's in kit or partially assembled form. Finding one of those would a real bit of treasure.
There are also japanese produced SKB'S and Asahi brands which are clearly influenced by the pre war Bsa's underlevers and Haenel break barrels and Webley MK3.These may have been made up until quite recently, though the design seems much older.
Very rarely are these for sale in the UK and would be an interesting piece to own.
The earlier pneumatics are perhaps the most interesting Jap guns - The Sharps Victorys and Pan Tagets etc.
Cheers
"helplessly they stare at his tracks......."