Crusty is right. It's a unique British threading standard from the 70's, now discontinued. But apparently some screws are still available in the UK from that standard, though no longer being made. This is the whole reason "cheesheads" (a term which predates Packer's fans headwear) are mentioned so frequently in Neve circles, because they fit the Neve parts of that era while no other screw/threading works as well. But you can adapt, and for just a few or a couple revolutions other thread standards can suffice...
I've been meaning to look into sourcing a large quantity of NOS BA screws and make them available in small quantities here... Would be easier for somone in the UK.
I haven't checked the new, current Carnhills to see what threads they are using. Would guess it should be metric now. Anyone know?
cheers,
t