Irem M72 incompatible with GBS-C?

TL;DR is the M72 known to have an odd/picky sync signal?

I have an M72 board (Air Duel) that I can't seem to get stable video output from. My setup is an Axunworks supergun with the VGA output going to a GBS-C board and then off to a display which works fine for a number of other boards (CPS I/II/III, MVS, F3, S16/32/ST-V, etc.) but the GBS-C doesn't like the M72 signal. The board is working in as much that I can hear it looping through attract mode and I can blindly start a game.

I have managed occasionally to get the GBS board into a state where it either passes through video briefly, albeit with quite a bit of tearing, or I just get a frozen frame from the board at the point I cycle to the RGBS input. Would it benefit from running it through some sort of additional sync cleaner circuit?

M72 uses 256 vertical pixel resolution, which is unlike other pcbs that use 240 /224.
So yes, it's possible the M72 pcb signal is outside of the timings that the GBS is made to handle.

Thanks, that could be the vital nugget of info. I raised a ticket with the developers of the GBS control firmware so they might be able to tell me if those extra pixels make it a yay or a nay.