They use #455, 6V blinker bulbs. I call them blinkers, so not to confuse them with larger, higher volt flasher bulbs used in newer pinball machines.
Most of the LED companies sell #455 LED blinkers too. I like the LED blinkers because they don't generate any heat to warp the black hole wheel, and they take about 1/2" less room under the wheel than the bulbs do, but I don't like the LEDs because they all seem to blink at the same time, and in recognizable patterns. I wish someone would come up with a more random blinking #455 LED blinker.