hello BIGBEN,i don't own a sa-20 but i was looking around and i see that the sa-20 uses spacers between the receiver and the butt stock. i,m not sure if this is used for the LOP or height. like LES said you might want to contact mossberg about it.
also saw a weatherby sa-08 butt stock will fit. again ,i don't own either shotgun. just thought you could widen your search.