When I got my first 2 pinball machines, both EMs - Bally Bon Voyage and Bally Monte Carlo - they did not have any decal inside the diamond on the coin door. It was just empty. I then got Strikes and Spares which is early SS and it does have the typical Bally decal.
As I am working on them and fixing them up, I thought maybe I should get some of the decals and put them on there. But then, when I started looking at a bunch of photos of other machines of that era I noticed that the vast majority are without decals, just plain coin doors. And, the picture on the flyers also does not show the decal. That includes Strikes and Spares as one example shown here. I noticed that for a later SS Fathom the flyer does show the decal on the door.
Was there a specific era where the decals came in and then were put into common use? Or is it one of those situations where it was kind of randomly done by the factory? Or maybe, an option to be ordered?
So my basic question is, should the decal be put on the coin door for the older machines, or left off if you want to aim for keeping originality?
Maybe I am overthinking this, but this is what I do, it is a mental sickness.