[The Mekon (John Briggs)]

Not sure if the title conveys what I want, but I would like to know what thread the small securing screws are on items like eyepiece holders, diagonals etc. In particular the GSO/Bintel types that are made in China. They are around 3.7mm diameter and I dare not run a thread die down one (they are too short to start with).

Reason for this request is that I have lost one from my 35mm Bintel 2" extension and would like to obtain replacement.