TPU caps for scopes, binoculars, cameras, etc. The mm number is the size of the cavity, and these are meant to stretch a bit, so for example the 42mm print is ideal for a 42.5mm-43mm ocular bell. If you print with a non-flexible filament, you will need to resize. Use randomized seams and concentric fill pattern in your slicer, if you want it to look like the photo. Using a standard fill pattern doesn't look as nice, but does eliminate the tiny hole in the center of the cap, making it more weatherproof.