I have an HP41C SN1926A00137 which seems to be a pretty early model. It is not operating but the guts appear to be very different than any 41C I've seen
It also has a plug-in module in slot 3 which is nothing more than a DIP switch (marked "K40"). The DIP switches (qty-8) are wired into 4 small circuit boards (23x12mm ea.) standing vertically on the left and right sides of the battery position.
The machine has some corrosion damage and does not work, probably due to the corrosion. I'm wondering if anyone knows anything about this weird bird and if its worth spending time on getting it going.