Oh, so the belt is supposed to ride against the wheel but not around it?
It was completely off and just scratching around in there when I opened it up, and it fit so neatly on the outside of that wheel I just assumed that's where it was supposed to go! I found a photocopied manual online that showed this area of the machine but the photos were so dark I'm not really surprised I got it backwards. That makes sense, though, because the way that wheel attaches to the machine is with an open slot and it seemed a little weird it would be pulling against something attached that way. If the belt is on the other side of the wheel that pushes instead of pulls and that makes more sense.
I will switch it next time I have her on the table. Thanks for letting me know!
I still can't figure out the purpose for that extra spike. I'm convinced it's not original at this point, but I still can't fathom what it could have been used for. Mystery spike!