PS- What is an "ET machine"?
ET means Energy Transfer. It was used on competition machines and provided 6V alternating current to power the lights. The original lighting was weak so ET guys would appreciate an easy upgrade to bright LED lights if possible.
A non polarity conscious LED bulb sounds like it would fit the bill because it implies that it will work with + or - ground systems. Since AC power continually changes from + to -, it's possible a LED bulb would work but I don't think anyone has reported on this yet. Magnetoman may at some point report his findings because he is curious about the possibility himself.