Oh gosh, I remember that rack and I remember those cards - but I can't quite remember what they were for!!! I think it was just, as you suggest, to switch those meters from normal internal desk connections to an external source. It should be reasonably easy to rewire to take them out of service, once we've worked out which wires it is...
I'll send a PM, and let's see if we can sort this out over email.
Do you know which vehicle this came out of (at least when it was made)? I'm wondering if it was the BBC Transcription Vehicle that I helped install in 1986 (I think), the "AFL submixer" sounds familiar. But we did supply other desks directly to the BBC for them to fit into vehicles.
Cheers,
Andy