Quoted from frenchmarky:

One little trick I did with LEDs on an EM -- on my ‘75 Fast Draw I have all 44s but I changed the ten bonus light inserts to warm standard LEDs. The game has Gottlieb’s ‘rapid-fire’ bonus countdown where regular lamps barely have time to light up at all because the count goes so quick, always thought it looked duddy, virtually invisible. With instant LEDs the effect is tons better, you can actually SEE the countdown!
I did need to add a resistor between those lamps’ common ground frame and it’s ground wire to eliminate visible flickers of the LEDs from parasitic wiring voltages when coils were firing off, which reduced their brightness just a bit but still look as bright as 44s/47s.