If I remember correctly 3BA screws were used on socket /switch box's before they went metric.
As for imperial nuts, bolts, screws I have thousands of them still, from whit-worth, BSF, AF, 2BA,3BA,4BA,6BA,8BA and some smallest ones used in watches etc. and some very large 1" Dia ones.
If I take something to bits I save the nuts and bolts
I have also a good range of taps and dies in my workshop .
One of the things we did on my apprenticeship in 1964 was to cut screw threads on a lathe .