Interesting question you have raised regarding the “size” of the tripod screw hole.
I hadn’t really looked at it any detail as the baseplate is part of the camera and I was more interested in the lenses.
I have kept a large database on the SERENAR 50mm f/1.9 lens, but it has been corrupted where I recorded whether the lens was in “ft” or in “mtr” due to me doing a copy/paste of large sections in my Access file. Probably why I hadn’t mentioned metric 50mm f/1.9 lenses as I hadn’t recorded any up to 2008.
Generally, the scale was in the British/Imperial measurement of “feet” which at the time was used by many countries but now only three countries are out of step with the rest of the world who have embraced the metric system.
I checked my five model II-Bs that I own and two of them have the 3/8” tripod thread hole (EU) while the others have the ¼” (USA) thread hole. One of these baseplates has the MIOJ mark as well as the 3/8” hole (#29470) and the other 3/8” baseplate is unmarked and that is from a very early II-B (#25015) which I was told by the previous owner that it was sent to a military big-wig guy in Hawaii direct from the factory hence the lack of MIOJ markings. One of those baseplates with a ¼” hole is on camera #25696 which is in between the two # above. As well as the ¼” hole the baseplate is marked MIOJ and has the <CPO> mark BUT in Japanese characters or ideogram. Page 24 to 31 in my book explains all these markings.
SERENAR 50mm f/1.9 lenses would be unusual to find with metric markings and as to its value that would be up to the seller to describe it as such and to the buyer if he/she is willing to pay a prime price for it. Canon Co were following the European form of measurement (influenced by Leica) to a point as they used millimetres (mm) to identify their lenses but feet (Ft) to identify the movements for focusing.
The main market at the time in Japan and later Korea was the Armed Occupying Military forces who at the time all used the British measurement in their respective countries.
I checked my ten S-II I have in my collection and only three have the ¼ “tripod hole and I also checked my “Canon Camera Holders” and all of them have ¼” holes. Maybe Canon were forcing the buyers to purchase Canon “Camera Holders” for the tripod mount as many do appear on the market but rarely with the MIOJ mark.
On the number of 50mm f/1.9 lenses produced, an official Canon Inc data sheet I own (acquired after 2008 when I produced my book) mentions that 26065 units were produced.
Looking at a model II-B database I have been keeping for some time I notice that 3/8” tripod holes are on early models while later models have the ¼” tripod hole.
So, if your model II-B has a serial # before #33210 it could well have a 3/8” tripod hole? Because after that # most I have recorded have ¼” holes.
Thanks again for an investigating question, Cheers Peter K