I've seen rear and front lens cap saying the focal length on them. The idea is great so you can tell what the lens is when they are in the case. The problem is when you are in a job, you swap lenses around and you will just put on any lens cap and put it back in the case and having the number on will just confuse matters. Besides, you get to know your case and where you keep which lens anyway. I like my caps uniform to the point that I get canon lens caps for my sigma lenses so no matter what cap I put on, they are all the same.