Well folks, we have our answer. My key so puller came in just now and made this a priority. They don’t have the Alps branding on the switch tops, so this means they are Mitsumi? These are pretty quiet to type on, and to be honest feels a bit mushy for my taste - could be due to age. There’s some resistance in the key presses that I’m not used to, which could be just because I’m so used to Apple’s chicklet style boards.
Anyway, I still think this is a cool collectors item. I made a USB adapter and have been playing with it for a couple of days.