So while this looks like Machine Doubling, why is it not, Machine Doubling? Note the bottom of the mintmark looks like MD, but the bottoms of the digits are not looking like that. But as to the cause of the RPM. The area you are seeing that is the RPM was probably the first tap of the punch. Keep in mind that they are hammering this punch into a hardened die. So to create the depth can take several hits on the punch. The second strike was slightly above the first strike. And because of the second location, it hid the first punch, that probably would be been hard to see, below the second location of the punch and proceeded to deepen the second location. But we spot it! That is why it is so weak, as it was the first punching.