As already mentioned, a lot of features were used over and over in different ways on different titles. Take for instance 500 (or 50 in the 60s) point scoring. 100 point relay activated by the 5 cogs on the score motor cam. Others have done a great job explaining EM logic here, but again, just build on one circuit, then add others. Before you know it, you have a late 70s EM that physically couldn't hold more equipment!