A lot of those Williams control panels are pretty similar. I believe they are nearly identical shape-wise, just different controls. So it makes it easy to swap games between machines. I think that is why the multi-Williams pcb from jrok is so popular. You can put it in just about any Williams cab (like Stargate or Joust) and be able to play them all (assuming you set up your control panel appropriately).
But, why a Joust overlay would be on a Stargate CP, that is a bit baffling. Unless, as you mentioned, the operator just modified it.