The big IC i suppose is ym2610 and the quartz is for 8MHz so i still think it is OPN (ok, OPNB) sound interface.
Nice sleuthing, but no correct answers yet...
YM2608 OPNA interface? (That would be pretty sweet..
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Or it's an Y8950 on there...
Anyway, 256K RAM on there, usable for both..
YM2604?
Pin-pon, ren takes the win!
Indeed, Pencioner did well linking the pin count and 8 MHz crystal to an OPN series chip, but the final clue was the 256K RAM chips in the top center of the board. The YM2608 OPNA supports sample RAM, whereas the YM2610 OPNB only supports ROM. It’s a prototype board that was sent to me by Leonard Oliveira, designed by him and Fábio Belavenuto (very grateful for that!).
I think that puts you ahead of Meits again! (And me on the same level as Johnny Hassink! For now...)

Cool! 
I identified the TI SN74LS04N hex inverter as well (and know those exist now
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Is that an own build proto board? (must read better - you described that in your posting) Some other day I thought to myself that like the only thing missing from MSX sound-expansion wise, was the OPNA - it's an awesome chip, got to learn some very awesome PC98 music this past year 
Where's that ribbon cable connected to?
It’s connected to a cartridge that takes care of the I/O decoding logic, so the attached board can focus just on the sound chip.
Thanks for the points Grauw!
