>> Instead of running a test, Mel kept incrementing the value of the (A) field, as described in the story. This eventually led to an overflow of the entire register, provided the index register bit (X) was on – exactly as Nather remembered. Using 101 as the TBC opcode yields the following sequence:
>> The overflow would toggle the BCU on, causing the heretofore ineffective TBC to transfer control to the address in the (A) field, which was 0.
Wow. I kind of waved my hands at that solution, but you worked it out almost in full (short of actually coding it ... I guess there's no emulator for the RPC-4000 around? Someone should go and write one seeing as the architecture is mostly in the manuals and all. Someone... else >_>). Good stuff!
Btw, if you're looking for translations to more languages, there is a Greek one here:
https://greektrans.blogspot.com/2010/06/mel.html
Complete with Greek-language annotations.