Thank you 'Zeotrope'. Much appreciated.I’ll try to measure it tonight.
Right, the central metal part of the platter is lower than the rest of the platter. The washer is at the same height as the main platter. When you screw down the clamp, it must press the label part of the record into the washer - it may even make the label concave (as viewed from the top).
What I found with lacquer records, which have a metal core, there was a need to really tighten the clamp on one side — too much for my liking, so I removed the washer and didn’t use the clamp. But that does affect the sound negatively. I’ll try with the clamp very loosely tightened so it acts more as a weight…