Not sure on the 303 compatibility, but the Beretta MicroCore pads that Stoeger, Canada has will fit 24 year old 391's and 686's (or the A400's). Just have to get the correct sized pad for a field stock or the larger "sized" competition stock and spec the pad thickness you desire. They are a good pad and much lighter in weight than the old rubber Beretta pads (less than one-half), which I like on either a field gun or target gun.
The pads I am referencing all have the "Beretta Bumps"....one round and one oval plastic protrusions that index into mating holes in the butt stock and are centered around the butt pad screws.