Sorry to bother you people, but I got myself stuck on this one. After some intense googling I only found one post, on reddit, where somebody sold a replacement crystal for these that was NOS, that stated on the package that it was 30.6mm.
I'd greatly appreciate it if somebody could confirm that 30.6mm is the right dimension. It's a high dome acrylic with a tension ring. This is an N0 watch, made in the 1970s. I have polished the crystal, what can be seen now are microcracks in it.
P.s I can't open it to measure it because my new caseback tool has not arrived yet. Coronavirus craze messed up my country badly.
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Thank you.