The RPC-4000 (the "big brother" of the LGP-30) had each instruction specify the next instruction address. I believe this was to allow for optimizing your program such that the next instruction would always be right under a read head on the drum when the processor was ready for it, because if you missed it, it took a whole revolution of the drum to get back to it (kind of like a cache miss).

In any case, it seems that while Mel wrote lots of code for the LGP-30, the actual hack in the story involved code that Mel was porting from LGP-30 to RPC-4000.