It could not have been proofed in 1926 for a couple of reasons.
--They stopped using fractions in the barrel weights in 1924.
--The gauge & chamber length is in a diamond, again that stopped in 1924.
--The date code is a "b" which is 1923.
The barrels were done by Joseph Cap, the receiver bears the trade mark of Alphonse Forgeron.
http://www.littlegun.be/arme%20belge/artisans%20identifies%20e%20f/a%20forgeron%20gb.htmHere is his registered trade mark from a calalog cover:
Unfortunately, I do not any of his catalogs, so I can not help with the model number.
Forgeron is also known to have used Cockerill barrels on his guns. Forgeron, while not well known here, was one of the top gun makers of Belgium when it came to quality guns. He did business through Europe, including Poland.
Some other Forgeron's
Pete