Showing my age here, but automations way back then most often used reel-to-reel "spotter" machines, usually Ampex. The decks were retrofitted with an electric eye sensor, which was hooked up to a "counter" and logic relay system. Clear "windows" about an inch long were created using nail polish remover to remove the oxide on the tape, and the system would literally count the windows going by as the relay system rocked the tape back and forth to find a particular cut. A bank of slide-switches was used to select among about 20 positions on the spotter reel in a step-by-step fashion. The system itself had no idea what was being played, other than that, say, position #12 was next on Reel 3. Logging was done using slow speed tape. Setup of a day (or less) of commercials was very tedious and easy to screw up. On the other hand, the playback quality was much better than carts. But, let the counter not trigger on one one or more windows flying by during REW or FF, and you might find the tape completely wound off. Frequent head and sensor cleaning was a must. The next event was triggered by adding a 25Hz tone in the left channel, which could be done either during or after recording the spot onto the "spotter" tape. You'd start the tone early enough to compensate for the time it would take for the oxide portion of the tape to get to the playback head, a little more than a second on Ampexes.
JJ