Agilent Technologies 53131A Assembly Level Service Manual page 173

Chapter 3 Replacing Assemblies
To Remove the Rear Terminals (Option 060)
3 Loosen and remove the 2940-0256 BNC nuts (H3) that secure the coaxial cable(s)
to the rear chassis, using the 14-mm deep-socket spin tight.
4 Pull W2 and washers (H6) out of the rear INPUTS opening(s).
5 Pull the connector-end of W1 and washer (H6) out of the rear INPUTS opening.
6 Plug opening (s) with MP9 (6960-0159) if you are not going to re-install Option
060.
Assembly-Level Service Guide 3-23