Here are catalogue pages for the some of the many signal conditioning boards that were made.

Arcom originally designed a 50-way connector pinout, but another company made a 60-way superset which they named the Signal Processing bus.

If designing new boards, I would use the 60-way connector because has an even number of pins either side of the board centre line. If using the 50-way connector, place it sits in the subset of the 60-way connector. This will not be symmetrical about the centre line, but that helps keep cabling between the two types free of bends.