just to elaborate here
what he speaks of, MK1 revisions 1.0-4.0 shipped with the Midway Y-unit boardset (like Smash TV, High Impact Football, Total Carnage, a slew of others...) which uses its own kick harness.
then, at revision 4.0, Midway switched to the newer T-unit boardset (like NBA Jam, MK2, and uh... that was it really) that uses the same kick harness as MK2. revision 5.0 was only seen on the T-unit board, so if you have a 5.0 it's a T-unit, and actually the game will say on the boot screen if it's T-unit or not.
I've found the easiest way to differentiate the two is by the size, the T-units are larger. the Y-unit I've found has the name of hardware designer "RAYMOND GAY" silkscreened in the edge. possibly the easiest of all, is the Y-unit's backup battery is directly next to the 2 banks of dipswitches, whereas the T-unit has a couple of white headers (I think these are for extra coin door wiring) below the dipswitch banks.
contrary to what's been said here (sorta) MK2, (U)MK3, and MK4 all use the same kind of kick harness -- only difference is MK2's factory Player 3 harness could be missing the wires that would go to the Run buttons. it's not difficult to mod a factory MK2 kick harness to allow the runs, but as per what's been mentioned above, make sure you put the wires on the right pins.