I'm 90% sure the middle saucer is supposed to kick into the upper left saucer. I've seen it on multiple machines, including my own (though sometimes it's not quite strong enough). It makes sense (to me, anyway) with the rest of the design of the machine. Otherwise, that upper left saucer is pretty pointless.

I'll echo this question. I have a Mars Trek that I bought all beat up and it is missing the spinners and spinner bracket. (Which is the excuse I use as to why I haven't tried to get it working yet). To my eye, it looks like the same set of spinners but I haven't taken them off Prospector to measure.