This is a M42 rear lens cap designed specifically to cover/protect the deep rear protrusion of an M42-mount KMZ / Zenit Meteor 5-1 16-69mm f/1.9 16mm Cine Zoom Lens. To my knowledge the Meteor 5-1 has the deepest rear protrusion of any M42-mount lens. The lens cap, 100% coded in-house by me, is styled similarly to tall PL-mount caps you may have seen for cine lenses but it's for M42x1 threaded lenses instead.
This cap should also look nice on your other cine-modded M42-mount lenses that are tricked out with 3D-printed follow focus gears and whatnot, and given that the cap is so tall, it should accommodate just about any M42-mount lens (Helios 44-2, Takumar, Yashica, Mamiya, Fujinon, Vivitar, etc.) as long as it doesn't interfere with any tabs, pins, or flanges I didn't consider.