So a few years ago, Classic Playfield Reproductions did a run of 30 Sexy Girl playfields.
They said in their brochure, that they were designed "to do a complete conversion of a Bally PLAYBOY" and that made sense, as the original 1979 Sexy Girl was a drop-in kit that converted a Playboy.
But if you did not look closely (as none of my clients did), you might not notice that the CPR playfields were routed only to accept Gottlieb hardware, not Bally.
So to do this conversion, you would need to kill a Bally Playboy AND a late 70's Gottlieb pinball machine to harvest all the parts. You would need the Bally wiring harness and boards. The Gottlieb coils were different voltages, drop target cages were different....sheesh, a pinhead could easily bite off more than he could chew here.
Since I had 3 Sexy Girls to build, that would mean I'd have to kill 6 pinball machines, 3 Bally and 3 Gottliebs.....
....or I could convert the playfields into a pure Bally system, making it 100x easier to build, wire and service in the future.