Agh. Soviet GOST standart claims that DIN objectives with D 20,2 mm thread are RMS standart (1936 version of 1858 standart).
There are also existed 25 mm Leicas and 27 mm, 20 mm & 19 mm Zeiss and co objectives. Most of those are antique, but I have fairly good 20.2 Nikons, Zeiss and LOMO objectives. All those are for 160 mm microscope and has different length.