And with the new ribbon; it types again! (Skips though. A lot.)
The old ribbon was completely out-of-ink. I won't say 'dry' or 'empty', because I do suspect the ribbon to be responsible for some of that mouldy-smell. It had been folding double length-wise, winding onto the spool folded in half. This of course meant it wound to a larger diameter; so it had become firmly stuck inside the spool-cup. And when it was well and truly wedged stuck, my guess is that the ribbon-reverse also didn't work anymore. It then probably spent a decade or more developing that green/black layer that glued everything even tighter. Yuck.
But all that is now cleaned. And disinfected with a good helping of IPA alcohol on the bare metal parts (taking better care now to steer clear of the bakelite).