Here are a couple of devices I dumped ROMs from over the last few months.
First, the EPROM from a Xetec C-5181 printer adapter for C64: https://revenant1.net/xetec_c5181.bin
Based on these photos
, I identified it as originally being used for the "Big Blue" thermal printer, which as far as I can tell is compatible with Epson 9-pin serial printers. The adapter itself runs on a CDP6805E2E, with port A connected to both the C64 serial bus and the printer.
Second, the mask ROM from a Casio CTK-551 keyboard: https://revenant1.net/ctk551.bin
This one uses the same main board and I/O board as the CTK-531 and CTK-541. The only difference is that each higher-numbered model has more inputs hooked up (velocity-sensitive keys on the 541/551, and pitch wheel on the 551). There are scans of the CTK-541 service manual w/ pinouts and schematics floating around online.
These all run on a "Casio GT913F", which I can only assume is related to the NEC µPD912 and µPD914 chips found in other CTK models. I haven't been able to find out anything about the actual architecture, though.
There's various (byte-swapped) ASCII text throughout the ROM, including for some test mode that I don't know how to access. The instrument samples start at 0x5A800 (or thereabouts); the samples don't seem to be straight-up PCM, but you can listen to them as such and still get pretty recognizable sounds.