Diodes on the lamp sockets were used on Bally games that had dual phase feature lamps. They were rare in the Bally-35 architecture - Gold Ball was one game that had them. This became standard in the 6803 games. It was essentially a simple, cheap and clever way of being able to double the amount of feature lamps the platform could have.

I'd say your '84 EBD has leftover sockets intended for dual phase games. The only effect those diodes will have on your game is the lamps may be slightly dimmer since they have a voltage drop around 0.6 volts. The playfield lamp boards with the black twist socketed lamps don't have diodes right?

Since EBD is a single phase feature lamp game, those diodes on the lamp sockets are not needed.