I think they used the standard #44 or #47 bulbs. I have a King of Diamonds and for the roto-target I used the Cointaker flexible LED lights (warm white). Really lets you target the light where you want and nicely brightens up the roto-target area.

Flex_Light_(resized).jpg

I used to own a Domino (good game) and think you could do the same thing with this game.

Oh that looks like it might be a #555 wedge base type. Looking at the pictures on IPDB, you might be better off with just a replacement for that style (either incandescent or LED). The flex option works best when you want to 'work' the light around an obstacle and shine it on/under something. Another good example is for kick out holes to move the light around the kicking mechanism.

There are a few choices from the automotive line of GE miniature bulbs. Look up GE 561, GE 7443, and possibly C656. There are also bulbs that look like fuses, Phillips 221-2 (T-3) might also work.
Hopefully there is a correct answer out there.

Added over 8 years ago: The 159 and the 259 will also work.

#159's wedge type.