Here is the toplevel schematic:
For runs 70482 and up:
For runs before 70480: PEN Prescaler (pdf) ~run70000 to run70480.
The 15-bit synchronous counter is here:
Now, since the LB500's have been changed to LB102's, we are no longer able to disable each prescaler input. The disable you see in the schematic is an historical remnant and can not talk to the LB102's from the frontend through the CAMAC crates.