What Recoil Pads would have been used on Belgian Shotguns is the mid 1940s to early 1950s ?
All of the shotguns made in Belgium
for that time frame, that I've seen , came new and originally with 'horn' butt plates as folks on the Continent didn't go for recoil pads like Americans do and still don't.
Browning shotguns are a good example of Belgian guns
from that time frame, answering your question, they all came new with horn butt plates and to ugly the gun up, the dealer would then add recoil pads on customer request in this country.
The piles and piles of used shotguns imported from the Continent,
for that time frame, that are sold cheap by 'large' retailers in this country are 98% without recoil pads,
but always have sling swivels and usually cheek pieces.
Folks on the Continent hunt with snow shoes/x-country skies and poles in the winter, needing both hands free, therefore the sling and swivels were born way back when and are still popular over there on shotguns.
I think the religion of the 'permanently attached recoil pad' is an American thing....
Regards,