Ok, so I took a flyer and bought myself a couple of HP-75Ds on eBay. Never intended to collect the 41/75/71B series, but what the heck? Price was right.
The 'putes came encased in 82718A modules, which I assume adds a modem and... what else? The auctions were "as is" with no supporting documentation or explanation.
I tried removing one of them from the 82718A - easy enough to do by removing 6 screws, but I see a pair of wires emerging from the side of the computer, leading to a mini-jack on the back of the expansion base. What is that?
The computers themselves seem to take classic series battery packs (and indeed power up and do things once such a pack is inserted). The base looks like it takes a pair of those packs. Only place I see to plug in an AC adapter is on the back of the computer itself - it has two prongs, and looks like it'll take the same charger as an HP-97, but I haven't tried.
I've looked through the docsets on the HP museum CDROMs, and while I find the owners and reference manuals, I don't really see anything pertaining to this external hardware.
Anyone got a clue they'd like to share? Thanks.