Very early Nikkor 50mm f3.5 collapsible. It came on an enlarger with a home made S to screw mount adapter. Turns it was a pre-production 50f3.5 for the Nikon 1 and it ended up in Hong Kong and later here in Vancouver. Supposedly the lowest serial number of any 50mm f3.5. I kept it for a while, used it on a S2 - but in the end sold it at Christies for a lot of money. This was all in the mid-nineties. I kind of like the dome shaped lens cap too!