Charles19, appreciate the photos, and my thoughts of the square peg into the round hole are have been trounced.
I question if this may have something to do with the new military contract, making it easier for troops to disassemble the magazines for cleaning.
Your original inquiry due to aftermarket base pads having the round port, appears that a source for the older style inserts will be needed, as I imagine they will be interchangeable.
Magazines made in Italy, are normally by Mec-Gar, but I have not seen Mec-Gar offer any 250/320 magazines commercially, so I don't know if they will supply repair parts for Sig Branded magazines they manufacture.
Send them an email at the following and ask...
Mec-Gar Magazines