I don't think the Game King/i960 platform is actually playable yet (the lack of Game King and I-Game dumps is a bit worrying too, given their age). Looks like MAME's Game King inputs are still hooked up in the DIP switch section rather than mapped to keyboard keys, which is usually an indication that the emulation is very preliminary. I couldn't get it to do anything useful whatsoever (although I have only tried the ms3 set as I don't have anything else to test right now).
MAME seriously needs some volunteers who know the first thing about the insides of slot machines, but virtually no-one is "alive" in the slot emulation scene aside from Jim Stolis (the PE+ guy) and a few UK/European devs who tend to work on other types of gambling games (UK fruit machines, European video poker, Cherry Master style 8-liners, mahjong/hanafuda etc.)