There is only a passing reference to this mdel in the MoHPC. Here are some pics:

http://www.decodesystems.com/hp9831.html

What do you want to know about it (I have one)?. The hardware is the same as an early 9825 (with the system ROM module that slides in the right hand side under the keyboard). But those ROMs contain BASIC not HPL. And the keyboard has a few different keytops (it's the same arrangement of switches, same electrical matrix connections).

So will the 9831A system ROM cartridge work in a 9825A, or vice versa?

I believe it will. I've been told that that's what people at HP did -- they had one machine on their desk and slotted in the appropriate system ROMs.

I belive you have the HPCC schematics CD-ROM. If you look at the 9825 keyboard schematics, it lists the (few) keys that were different. It also shows the 9831 (and not 9825) ROM module schematic, as that's what I have...

What about making EPROM copies of the 9831? It would be extremly interesting for all 9825 owners.

Remember the ROM chips in these machines contain the address decoder and address latch circuits (it's a multiplexed address and data bus). You couldn't just replace them with EPROMs, and you can't read out the HP chips in a normal EPROM programmer.

It probably wouldn't be too complicated to make an interface to normal EPROMs, and, indeed, reading out these ROMs is on my to-do list...