> They borrowed an acoustic coupler—a forerunner of the computer modem—and connected it to a nearby pay phone

The acoustic coupler is mounted on a modem, and is just the cradle where you rest a handset. The device is not a forerunner of a modem, it is a modem.

Almost. A modem sometimes had a phone jack as well as a coupler, for those cases when the handset was hardwired into the phone and the phone was hardwired into the wall.

We tapped where we could and we were happy. Bonus points if the rotary phone had a lock on it and you dialed out by pulsing the hangup switch.