According to the FWB exploded diagrams for the two guns both but pads are fixed to the butt using screws into captive fixings sunk into the wood of the stock. However, from memory they can be unscrewed from the stock so could be removed and plugged and different fixing spacings used if necessary.

It's just possible that a trawl of local shooting clubs might turn up something as there might well be a few old FWBs still in use that have had a but pad upgrade and the old one is sitting in a cupboard somewhere begging for a good home.

Either way, good luck in your search, Vic Thompson.