These are the dimensions that count: The entrance pupil of the camera's lens, when zoomed down to the full wide angle position (focal length = 7.1 mm, equivalent to 28 mm for a 35-mm "full frame" film camera) was determined to be at 56 mm FORWARDS from the camera's tripod socket. The 38 mm distance in turn is what places the rotation point underneath the center line of the lens.

The little lock pin engages with the corresponding hole in the 828's base. With the thumbscrew turned down hard it effectively prevents any unwanted rotational movement of the camera here. What I hadn't known before, by the way, is that these pins and their sockets are shaped conical along their axis rather than cylindrical, exactly for this purpose! Both this and the 14 mm distance between the two sockets are an international industry standard.