Guys, the answer is so simple... I am really surprised noone knows what the rounded rectangle dimple/hole is for... It's there for a square bubble leveler! (look at the picture) This leveler is my favourite accessory because the bubble sits on the center at exactly 6,5 angle!!!
On one hand I understand this confusion. Here in Europe, every Funhouse and Addams family (and possibly others too) I have seen, have these black square bubble levelers, whereas american playfields don't have them (mystery?)
P.S.: If someone knows where I can buy such a leveler please p.m.