Never seen that extra PCB, with the three blue coax cables, in any 5313xA. What option is that?
There are a few interesting things in this 53131A
I am not sure if this board with the blue cables is an option or was standard in the early counters.
The small blue cable is to take the 10MHz output from the Option 010 (High Stability) board to the main board output.
The little circuit board withe the 3 blue cables has a relay and can switch the 10 MHz output to two different locations on the main board:
Location 1: EXTERNAL 10 MHz OUT
Location 2: Ref. IN
May be it is for self calibration purpose? I am not sure and did not read up on it.
Interestingly this counter has the high stability crystal option 010 but there are no power supply cables going from the power supply to the Option 010 board, like usual (J2 on PCB not populated).
Instead it is powered by the ribbon cable.