"On the flip side, why would a person or establishment OTHER than Al's Cardroom go to the trouble of buying a customized rubber stamp and making homemade chips this way?"

Maybe so they can sell them to unsuspecting collectors?
This is a very modern chinese made diamond mold blank. The injection mark gives that away. Chips cost 4c each, rubber stamp about $15.

Only way to verify for sure is if someone went to the joint to take a look