Reading switch labels on a Genco schematic
A small number beside a switch is the relay it sits on (23 means relay 23). A larger number such as 1515 is a unit, and the code under it gives the switch and layer, for example 1515/1B for switch 1 on the bottom (B) layer; T is the top layer.
Systems
Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part Two, Clay Harrell (pinrepair.com)