I have re-read Tim's review on the ciné lens adaptor and he references the focus plane mark on the PL adaptor. Can someone verify that this is the inscribed channel that encircles the adaptor approximately at the front glass element in the adaptor? I see no other marks on my adaptor.

I would think that a focal plane mark should be on the camera body.

Because it has optical glass in the adaptor that "refocuses" the S16 image for capture by the 1/3" CCDs, the plane is inside the adaptor.

Further research LEADS me to believe that it is in fact where the groove in the adaptor is.

On the product overview page at the JVC site it describes the focal plane as: "Also, the 52mm focal plane is clearly marked as a groove on the barrel."

Yes it is the groove on the circumference of the adapter. It didn't exist on the prototype I tested when I wrote the article but it was my big request of JVC and they added it for the final product.

Thanks for the clarification Tim. I'm likely going to be using it soon and I just wanted to find out where to measure from to set up backfocus.
PS. My serial number is 59. How cool is that...