I used Sega Ages 2500 Vol. 23: Sega Memorial Selection for the PS2 and cheat engine debugger to do a rom compare of the MAME romset. I had to grab the z80 rom addresses in the PS2 emulation.
The romsets are identical with the exception of 1 hexbyte of data in ROM U4
This is what I found. u4.bin at hex address 34A: value 28 is cocktail 18 for upright.
You can use the MAME debugger and change the byte on the fly back and forth and the screen with flip or not flip on the second player.
It's just a matter of someone else verifying it other than myself.
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