Hi Todd,
I'm not sure Carnhill ever switched to 4-48, but have a look at the specs for 4-48 and 6BA:
6BA major diameter 0.1102", pitch 47.9 tpi
4-48 major diameter 0.1120", pitch 48 tpi
perhaps this explains why they're pretty much compatible.
(are we at 3 pages yet? :grin: )
Take care,
M.