When those machines were new they were like buying a new car - each feature was optional. Base machine would have one coin slot, next level would have two and top level would have 3. Companies charged extra if you wanted a nickel or quarter mech on a dime machine. If they didn't have a nickel slot they didn't need that relay.

Same as them selling two player & four player versions of the same machine (with different name and backglass). The 2 player version might have been $50 or $75 cheaper because it used less parts.