What can I say?
It's a rear lens cap for Nikon fit lenses - so it fit over the rear of Nikon fit lenses.
If you want a rear lens cap for your Nikon fit lenses then this is the one to go for.
Seriously I bought a 50mm Nikon lens and it came with a nasty, flimsy, white plastic rear cap - I bought a few of these to replace this cap and to keep a couple as spares as I am always losing caps. It is much better than the caps you get on Nikon fit lenses from suppliers such as Tokina and Tamaron.
Well worth the small investment.