I get some interesting lenses to work on. This is a Nikkor 5cm F3.5 in a collapsible Leica mount that features the old-style F18 DOF scale. The lens uses the newer style F-Stops. Rather than a "QC" marking, meaning 4 elements-coated, this lens has a Red Dot. The last three digits of the SN are engraved on the internal fixture. The lens had a lot of internal haze, is coated. No tool marks were on the lens before I got to it, everything in TIGHT. It's much better now, it is constructed similar to a Canon 50/3.5 collapsible. This lens is most likely a prototype.