I got a lens from one of these and noticed that the elements were reversed from what I thought
so I had to remove it from the cell and flip the elements. Part of the reason I had to do this was
that there was only one, skewed air space tab. The newton rings were off-centre by 1.75 inches.
Removing the tab and replacing them with three equal-spaced ones reposition the newton rings
into the centre of the lens. There was an o-ring (which I got rid of)
that was impinging on about 2mm of aperture. It wasn't needed (since an objective isn't supposed to
be locked-in tight by a retaining ring anyway. There was enough thread to screw it down properly.
The o-ring was interesting. It smelled. It was a dark-green plastic but it smelled strongly of (kind of like) vinyl.
The ring was malleable, with little "memory."