I have a 50mm f1.8 e-series lens that I listed on ebay, then realised that it's missing a rear cover at the back - it's basically a disk with a protruding ridge that I think fits onto three metal flanges. I've found a picture of another one that I used to have that was complete so that you can see what's missing:
The complete lens:
The incomplete lens:
I think that someone must have removed it to service the lens and forgotten to put it back together afterwards. If I'm remembering correctly it's made of plastic. I've only ever used these lenses on a D90 and I don't know if the missing part engages the camera mechanism at all on older bodies, but it looks like it might.
Is it possible to get a replacement? And if not, can the lens be used on older cameras? I've asked Nikon UK but I suspect I won't like the answer...