For anyone who like's to use it, one important thing about use of 3/8" to 6mm Adapter screw with ring: the ring is a little bit curved. The convex side has to show to the 3/8" part of the screw to get the best friction. Concave side has to show to rotator top.
Thanks for the post and the welcome! In my blog post, I state the exact thing, convex to screw, concave to rotator:
Assembling The Rotator Unit:
The RD16 has a 6mm threaded hole in the center of the top surface. It also has a recessed area around that hole for a lock washer. The washer is conical, and typically in this situation, the screw is tightened down onto the convex side. The recessed area is deeper than the washer is thick, so when it is all tightened, the washer is below the top surface of the rotator and will not interfere with whatever is screwed down onto the M6 to 3/8-16 Thread Adapter. In this case, the RRS PCL-1.
I'm a mechanical designer, so I've designed many things like this.