Try to imagine something decidedly annoying that doesn't get much of your attention. That would be a good way to sum up automatic transmission shifters in classic Mustangs. We use them whenever we drive our Mustangs, and they get really sloppy through time and use. They get so sloppy that it's sometimes hard to know what gear they're in.
There's not much to an automatic shifter; a lever, cable, pivot bushings, an indicator light, a lever underneath, and a shift rod that connects the shifter to the transmission. When the shifter is shoved into Park, it pulls on the transmission's detented bell crank that moves the valvebody's manual valve and parking pawl mechanism. When the shifter goes into Park, for example, we're moving the manual valve to neutral while engaging the parking pawl at the same time. For Drive or Reverse, parking pawl is disengaged, and the manual valve in the valvebody is slid into the appropriate gear range, which directs hydraulic pressure to the right combination of band servos and clutch pack engagement to get us going. The shifter has detents, like the transmission, to keep it in the proper gear range.
Ford automatic transmissions prior to '67 didn't have a conventional gear-selection pattern. For example, the C4 Dual-Range three-speed automatic transmission used in '65-'66 Mustangs has two basic driving positions: small dot and large dot. The small dot position is actually Second gear for winter road starts. The large dot gets us started in First gear, with a normal 1-2-3 upshift. Ford called them Drive 1 and Drive 2, respectively.
Beginning in '67, Ford went to a more conventional P-R-N-D-2-1 pattern across all models to offer a more simplistic shift pattern. This is important to remember when searching for a shifter. For '65-'66, the shifter detent pattern is different than '67-'68. Both the shifter body and bezel are different as a result.
In '69, Ford simplified shifter design by coming up with one automatic floor shifter for just about everything. For '69-'70, it was die-cast chrome. Beginning in '71, it became lightweight, black plastic over the same shifter. Because the FMX transmission was decidedly different than the C4 and C6, it didn't have the neutral safety/back-up light switch at the transmission. It was located in the shifter instead.