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Dave,

The pins for custom signal connections could be ignored in an extender cable. Those pins are to allow the user to interconnect the rear in/outs between plugins. For ex, you could take the rear trig out from a function gen & connect it to the rear sig in of a counter.
You may have noticed that most "measuring" modules have a front panel switch which allows you to select Front/Rear input. This lets you monitor the FG's frequency without adding front panel clutter. Since you only use an extender for troubleshooting or repair, monitoring another plugin is unnecessary.

All these backplane pins are unwired from the factory.
Tek sold a little kit which had extra keys & a fist-full of jumper wires.

On the edge-card end (male), you will notice that there are several keyway slots. This allows the user to key the mainframe so that only a counter will fit into a specific slot, while only a DM will fit in another, etc. This ensures that the user could not reverse the 2 plugins which I used in my example above. It wouldn't hurt anything, but the rear connection scheme wouldn't work.
All TM500 plugins will have one key slot which would prevent swapping 5000 series scope plugins into a TM500 mainframe.
In addition, all DM's would have an additional key, all DC's would have a different additional one, all PS's still another.
By adding extra barrier keys, the user can prevent plugins from being swapped around & negating the interconnection feature.

The extender has to accommodate all these different possible barrier keys in the mainframe, hence all the edge-card slots.

I looked on my Tek-made extender. The 17 thicker wires go to the bottom 17 pins. All the smaller wires go to the remaining top pins.
I did not open up the connectors to see which side of the board they connect to.

HankC, Boston
WA1HOS