It just depended on the market. I'm not sure if any other English speaking countries got "Special Steel", but for sure the rest of the world got the phrase in French. Acier Special makes the gun more interesting, but your gun is a shooter and it doesn't affect the value. Thanks for adding more pictures. I'd still like to see the address on the top of the barrel if there is one. US barrels look like this:
Well, we know that your barrel came off of a World Market gun. Whether or not the rest of the gun did might be cleared up with a call to Browning. Browning generally has records of when and where their guns were sold. No record probably means they never sold it. Unfortunately, their historian, Glen Jensen, recently retired. I don't know if he has been replaced yet.
No. There should be two diameters stamped on the bottom of the barrel, one at the muzzle and one 8.6" from the chamber. The difference is the choke. So I've no idea which diameter is stamped on yours. Nominal bore diameter for the 16 gauge is supposed to be 16.8mm, but I don't know what the tolerances were at FN. My 1924 barrel also has a single number stamped. It's 17.2 and the gun patterns Full. I can only give the same advice I always hear when anyone asks about choke-- it doesn't matter what the choke says it is. Pattern it and see.