If you believe the patent docs, the reason GTB came up with that credit unit was because operators were supposedly complaining that people on location were cheating by picking up and dropping the back of the game, which was causing the add plunger of the traditional replay stepper to bounce. That would advance the credit wheel so that the game could be played for free.
In contrast, the design of the half-moon credit unit has both the add and subtract drive plungers mounted side-by-side, and they move in the same direction. If this unit is bounced, both plungers move at the same time, causing an add and subtract to occur together which leaves the credit wheel showing the same value, defeating the cheaters.
I never once saw or heard of this particular type of cheating on location. I was surprised when this unit came out, which "fixed" a problem we didn't have in the first place. (We had a lot of trouble with cheaters dropping the FRONT of the game, but that's another story).
I was always unhappy with this unit because it was so touchy, didn't hold very many credits, and frankly looked rather goofy to me. As Boilerman says, I think GTB must have had a lot of complaints about this unit because (according to IPDB) they did change back to the traditional replay stepper after Sure Shot.
- TimMe