Considering all bumpers are the same amount when unlit, I would suspect they use the same wiring up to the 1 and 10 point relays. The yellow pop bumpers light together, and the reds together, in this game, correct? That is the usual way. The way these pop bumpers work on nearly all older machines is say you hit one yellow bumper with your ball - the other one would fire as well. It was done to simplify things, all pop bumpers of the same set (i.e. same color, and they light together) fire together due to how these machines are wired up - saves extra resources and "logic" inside of the game.

Each pop bumper set likely has its own locking relay, so 2, which are locked on when the pop bumper sets are lit, and off when they are not. There is likely a make-break switch here that is like the following, this is the "off state" which would then switch when the relay is locked on:

emissueflipperparade2 (resized).png