Did Japanese manufacturers from the 1960s and 1970s follow any specific standards for the thread sizes of components such as eyepiece thumbscrews? I have been trying to match the thumbscrew threads on a 1960s Tasco diagonal with no luck. 3 mm is too small and 3.5 is too big. Standard North American machine screw threads have been no match either so far. I haven't tried small SAE threads yet, or (God forbid) Whitworth. I suspect they are an oddball metric thread but I haven't fund a match yet.
It is frustratingly like working on old Italian 2-wheelers. M7 studs?? Who the **** uses (or sells) M7 hardware? And then there are the little brass screws in the electrical terminal blocks. M2.5 - too small. M3 - too big.