Can anyone tell me what gets mounted to this spot? Assume it monitors flywheel speed for motor RPM but wasn’t present on my old transmission.

https://www.z1motorsports.com/cam-crank-sensor/nissan/oem-96-300zx-z32-crank-angle-sensor-p-4621.html

There is a reference to it in the '96 FSM (OBDII):

Edited August 20Aug 20 by AndrewG
Better FSM reference to part

Learn something knew every day on here 😃

Thanks Andrew!