And if you look at computers based on drum memory, then you probably didn't have a PC, the optimal place for the next instruction then wasn't the next address,

And if you look at computers based on drum memory, then you probably didn't have a PC, the optimal place for the next instruction then wasn't the next address, but instead how far the drum rotated during execution, at least in some of those machines the next instruction was encoded in the current instruction. Having RAM seems to be important to having a PC, that works well.