LOL Thanks Steve! What is your take on the age of my gun?
I think Drew's assessment of 'prior to 1877' might be as close as one could get with a Belgian pin-fire. That's already pretty good, considering that on the Continent the design didn't really change up until the end of the 19th century, and where the pin-fire remained popular. Unlike in Britain, which had a very short infatuation with the pin-fire, and where new action designs kept popping up, most guns in Belgium, France etc from the 1860s onwards followed the standard pattern, much like yours. There were some oddball actions with sliding barrels and such, but most followed the standard iron fore-end and long forward underlever design (why change it if it works?). Yours has thin fences, which, on a British gun could indicate an early manufacture, but on a Belgian gun it could have been made into the 1870s.
Trying to date a pin-fire gun without a maker's name and a set of ancient ledger books is a real challenge. British guns could have additional clues, such as patent marks, craftsmen's marks etc., but unfortunately my library is thin when it comes to Belgians guns of the period!