Yes, as I remember it from my days at SSL 1980-87, SSL sold extender card sets for the 651 and the channel modules.
1 The channel module extender card comprised of two PCB's, one of which plugged into the actual channel on the console busscards with a big PCB and then it extended out via 3 x 50 way ribbons to a smaller PCB which the channel module plugged into, in order to test it outside of the console frame. I believe that Recycled Audio, here in the UK has its own version of this for sale.
2 The 651 extender set was not sold a lot back in the early years due to the price but, as someone states on here, they were a mix of different extender cards which had (from memory) 55 and 65 way connectors. The idea was that you could raise (or take out) the 651 centre section within the console, plug in the right extender card and work on the card which had the problem.
Both 55 and 65 (if I am correct on this) cards within the 651 had different keyway gaps. The cards in the 651 had the blue (T&B??? ... Thomas and Betts??? or maybe I am wrong and they were Amphenol or something else) PCB upright connectors with a gap at different places where a metal keyway (block?) was fitted so you couldn't plug a card into the wrong place, if this makes sense?
Not many clients ordered the 651 extender set as it was quite expensive due to the different connector types and the keyways involved but, it shouldn't be hard for someone, these days to do their own version I guess (with help from others on this forum!).
Regards from Jim Lassen in the UK and hope you are all well and busy considering all this Covid stuff affecting our industry BIG TIME?