For many years, the manufacturers stressed 3 balls to operator to increase profit. Even back in the EM days. I remember playing a Space Mission on 3 ball when that was unheard of in the area.

Being in the industry, it was really stressed to go 3 ball and 50 cents at Firepower. Certainly true for Black Knight to be 50 cents. It was tough to go from 5 balls to 3 when You were an already established location.

For what its worth: I also purchased a Filing cabinet from the Service Manager's office when a Distributor closed down. A number of the Factory Manual Envelopes were stamped: 3 BALL . I believe these were mostly, if not all, Gottleib games. These titles were typically from the mid 70's and on. So, even back then, the factory was pushing 3 ball to the operators.