Yes, the diaphragm diameter cannot be measured directly, you always have to take into accout the magnifying factor of the lens.
The thing about the f/2 and f/2.8 of the Retina is no myth. I think Rick Oleson described the method of removing the stop on one of his fabolous camera repair pages - if they still exist, must be a decade or so since I last visited them.
I am sure it s 0.5mm. All older german cameras (and most, if not all, japanese ones) use metric threads, and I have reshaped a lot of filter threads with a 0.5mm pitch thread chaser.
I am owner of a PF2 (and former user). I cannot help you with this specific problem, only thing I knew that there were a lot of modifications during the manufacturing period (and sometimes even left-overs of older parts were used), so it s hardly possible to check by the s/n how your camera was assembled. There is a forum on Foca cameras. It s in french only but I think some of the members will understand english, so you might post your question there.
http://opl.foca.free.fr/index.php