Thanks for posting this and pointing me at the forum! Plenty of cool stuff to read here.
Anyway, I'm still not sure if my memory is faulty or if the assembler has really been lost to time. I'm pretty certain that what I used was definitely called the Melbourne Hose Macro Assembler, but I'm starting to believe it might have been something that some random person had just renamed that way.
I found
this interview with Nigel Spencer on C64.com that describes the development environment at MH back in the day, and says they used BBCs and XTs.
Quote:
Originally, we used Acorn BBC computers to program the C64 as they had much faster drives that also were dual disk and dual sided. They had the assembler and linker apps onboard on ROM's which meant they didn't have to load these programs from disk (we used the BBC ADE assembler back then). They were connected directly to the expansion port on the C64 to send code and data at high speed. Eventually that changed to the PC XT with a hard drive.
This doesn't mean there wasn't an in-house native tool on the C64 as well, but it's definitely filling me with a lot of doubt.
Anyway, if anyone stumbles over this in the future and knows more, I'd love to hear about it!