I have a Roleiflex 2.8C, the last Rolleiflex model with 10 aperture baldes. Last year of manufacture was 1954, I believe. After this, the Synchro-Compur shutter units that Rollei used came with 5 blades.
Minolta Autocord shutters seem to have gone from 10 to 8 blades around 1956? Not certain when the Seikosha MX shutter was replaced by the Citizen. After using a couple of Autocords with 10 blades, I sold the first one with a Citizen shutter within a month- couldn't stand the different look of the 8-bladed aperture compared to the 10-bladed.
Yashica- Copal- had 10 blades into the early '60s it seems before going to 5 blades.
Anyone know when Hasselblad came out with the first leaf-shutter model? This would give some credence to the auto-aperture thing as the lenses needed to stop down quickly. Any other camera making such demands on apertures around 1954-5?