OK, my thread gauges are a commute away, but my camera is here, so I unscrewed a black knob and it fits perfectly into the 3/8 socket in the bottom of various cameras and tripod heads.
I don't know what the reason might be, but I sincerely doubt it was a lack of tooling. My grandfather built Seafires during WWII, and although machine tools were hard to come by during the War (and aluminium stock like gold dust), both were freely available later as war stocks were wound down. Switzerland would have had no trouble getting hold of the required dies and taps. It's more likely that Koch wanted it to be possible to screw standard spigots (for flash equipment perhaps?) into the tops of the standards. I have never seen a Sinar publication advocating anything other then the systems of rods, but you never know.
I suspect the standard upright and its foot were made in one piece for stiffness. A socket would have been needed otherwise, and that and a bolt would have added to the bulk. The flared out portion (needed to provide enough friction to lock tilts) prevents the standards coming close together as it is.
As for the diameter, i suspect the surface finish on extruded rod wasn't good enough for smooth operation, so any upright needed to be machined to tolerance (another reason for making the upright from oversize stock). Then, my experience is only anecdotal, but the instrument makers I worked with at a lab in Berlin used to ream holes to nominal size and use slightly undersized shafts. I was taught the opposite in my (UK) school machine shop, but reamers come in undersize/oversize sets, so I don't suppose it's an absolute rule, even in ISO-land.
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