I had a similar issue with a 1961 Holdsworth.
From Doug Fattic https://www.bikeforums.net/21585236-post12.html
Advice was 5/16" BSF 22 tpi.
But it turned out I needed 5/16" BSCy 26tpi, which is an older standard.
Late 70s is probably metric, or possibly BSF, just thought I'd mention there is...