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Still feeling awful, but I managed to venture out to the shed today and do a bit of further investigation. I removed the small central lens from its centring (?) ring. It was snug, but not tight fit. They achieved that by wrapping two layers of scotch tape around the lens. The coatings look to me more like single layer magnesium fluoride on high refractive index glass, than broadband multilayer, but I can't prove that.
The diameter of the lens varies from a maximum of 50.17, down to 50.00, depending where you measure it.
I also had a look at concentricity of the two big spacers. The shorter one has a runout of 0.3mm on the inner diameter that defines the position of the small lens. The longer spacer is so bad that I did not even bother to measure it. Have a look at the two images. It is the same part rotated about 180 degrees. You don't need any measuring instruments to see how eccentric the inner machining is, as revealed by the varying width of the 45 degree step that was machined concentric with the inside.
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