Little did I know, there was a disk in the external drive (I hadn't hooked it up before today). It booted up to Finder 5.1, which did show a . after 512K. Yet, the model number is M0001W by all external signs. Maybe somebody upgraded the ROM at some point - I haven't opened it up yet. But I think this is a fortunate turn of events.

Alas, the spring is missing under my left Option key. :'( (You can see it sitting depressed in the picture.) I'm sure I can figure one out to replace it.

I gave the machine a good cleaning with a washrag, and it is very clean indeed, certainly less yellow and dingy than my Plus.