@Kroc Turns out the Master version is based on the C64 and Apple II versions. The sources very clearly go BBC Micro cassette -> BBC Micro disc -> 6502 Second Processor -> C64 -> Apple II -> BBC Master -> NES, with the Electron branching off the BBC Micro disc and merging into the C64 version. It’s fascinating - I’ll be writing a deep dive at some point.

Incidentally, I have updated that comment to talk about the 6510. Thanks for that! 👍