i went out and looked at a power master that came off my 70 that i replaced, i feel its original, but could be wrong, but theres nothing stamped on that pad but hard to tell, if there ever was anything. are you positive they were all stamped? im sure you know more about it that i do. also mine dosent have that sharp edge around the stamp pad like the one in the picture.
Machined half-moon area on front of master will typically have EB code and Julian date. Some are so light or weathered they are hard to distiguish...Some 409s lack the machined area, and some 409s lack bleeders, most likely these are later replacements.
Looking for used since I'd send an NOS unit out for rebuild anyway. Yes, the stamps were often quite light and sometimes excessive corrosion could "erase" the stamp. As Tommy said, if teh pad was machined, it did receive the stamp...if not machined, likely a parts store replacement.