This is a simple, but elegant, rear lenscap for M42 "universal thread-mount" lenses. The M42 lens mount was created by Zeiss in 1938 and saw common use in Pentax, Practica, Zenit, and other SLRs until around 1980, but has become popular again because M42 lenses are easily adapted to most digital cameras. The inside of this cap is an easily-printable thread compatible with standard metric M42x1. The outside is a grip pattern similar to that used on the focus ring of many M42 lenses.Print it; I used PLA. Despite the 1mm-pitch thread, no tweaking should be needed even with extrusions as coarse as 0.3mm diameter! As you can see, it looks cool in clear, too..... Aside from being a useful part, this was done as a test of my not-yet-released easy-printing metric-compatible thread library....