Do you have a picture of the switch matrix to post?

Slam Tilt is 53. If it goes away when you unplug the connector to the back box, then it sounds like a playfield issue. Check all of the switches in row 5 and column 7 for a possible short to ground. Look for the wires for the slam tilt. Since the slam tilt is optional, I’m assuming there’s an unused connector for it somewhere. See if one of those wires is shorting on something.

A shorted diode creates a rectangle on the switch matrix diagram — when two switches are closed, and you press a third, it triggers a fourth, all in a perfect rectangle. It doesn’t sound like a diode from what you are saying. But, to check diodes, they have to be out of circuit, or have one leg on the open side of the switch; you will get infinite resistance in one direction and continuity in the other. I wouldn’t start chasing diodes quite yet. Look for shorts according to my earlier post first.