Most of the time, these caps are taken off a line that is being up- graded or extended, removing or changing the need to compensate for power factor. So, they end up in the pole yard, set out in the rain and not helping a needy Ham Radio op at all. I picked up a bunch of them several years back not knowing what they really were.
Base diagram shows it as some sort of triode.
Using the wrong one is asking for an accident to happen. Make sure you know which is which. The AC rating is because they are motor-start units. They are used to provide an additional phase for two phase motors. Some newer versions use a plastic film dielectric instead.