This is somewhere in the FAQ, but to summarize: Keith loves you. He lets you install scriv on all the systems that you own using just the one license. So no need to buy a second key.
A further question. The clacky little computer arrived but doesn’t have a working screen, so I’ll have to send it back to America and get a replacement.
But meanwhile, I discover that it’s running MacOS9. I know it’s capable of running the early OS X - but where can I download or buy this?
The lowest version you can run and still use Scrivener is Tiger (10.4). You should be able to pick up a copy of this in the used market. Tiger is rated to run on some G3s (you can find the full specifications here, and it should actually be a bit faster than older versions which were less optimised.