Quite often it becomes necessary to replace screws or nuts when renovating an old telephone and a knowledge of the type of thread likely to be encountered can be useful.
Despite their continental origin, all Swedish Ericsson phones used British Association (BA) screws and all dimensions are imperial (inches).
This also carries over to Ericsson phones made in England but it may not be the case for those Ericsson phones manufactured in the USA.
Other continental phones generally used metric threads.
American phones have an American standard thread typically designated 6-32, 4-40, etc., although Whitworth threads also appear. But recently, trying to match screws in a Western Electric transmitter proved impossible and the same has been reported in other American transmitters.
American threads may be designated ANC (American National Course) or ANF (American National Fine).
One of our American members, Georg Ek, from Colorado, has supplied some information describing the so called American threads....
They are typically designated 6-32, 4-40, etc., where the first number gives an indication of the thread outside diameter whilst the second number gives the threads per inch (t.p.i).
ATCS Newsletters 9/84, 11/84 & 11/87