FWIW- I believe that those cases require the basic stamped key. Often those keys are universal. If you have any lying around that look similar to these, you may be in luck.
(IMO) I don’t really believe that any years of the “Pre-Plaza” era H-150s’ were any better or worse. You really have to take them on a case by case basis.
My understanding was that there were no absolutes regarding what type of magnets were used. Gibson was notoriously cheap and went with whatever magnets were the best deal from their supplier.