6 and 6A MPUs are backwards compatible without requiring any modifications other than swapping in the appropriate ROMs.

That label that says the board is experimental gives me pause.

I would carefully compare it to a production system 6 board for differences before attempting to use it. It might be a pre-production board.

As for the ROMs, they are labeled on the assembly drawing of the MPU. The six sockets in the lower left house the game ROMs and flipper ROMs. However, without labels, there's no guarantee that they are in the correct positions. To verify, you would have to read each ROM with a burner and compare them to known ROM files.

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I have done a few of the "experimental" sys 6 boards. I think they are essentially a 6A MPU that is still labeled 6. I think these came from BLACKOUT? Or whatever the game right before firepower was.

Best guess it was when they where playing around with getting rid of the data buffers.