My schematic shows paths through the match unit, 2nd coin chute, and through the thousands reels via the high score point adjustment.

When “L” fires, it travels through P1B,
to the first player thousands score reel,
through the high score adjustment plug,
to the ten thousands score reel,
then P1A on its way to the replay unit.

So when you have the plug set at 55,000, when the 10k drum is in the “5” position and the 1K drum is in the “4” position (54,000), a path is completed to “L” so that the next time “L” fires (you score 1,000 points, moving from 54 to 55) the Add Replay Unit will fire.