Glad you found the right pins hehehe.

Your measurments on Z26 indicate the slam switch (or mod) must be stuck on.

When Slam-Switch activated :
Z26-13 = Lo.
Z26-12 = Hi, therefore U5-15 also Hi.

and so the opposite is true...
In normal operation (slam switch NOT activated) :
Z26-13 = Hi.
Z26-12 = Lo, therefore U5-15 also Lo.

I'd now be looking at your slam-mod, or the switch itself, shorted diode on it or damaged/bent switch ?? ??

MM.