Ok, maybe the question can look weird. And actually it is. Let me explain further: a friend of mine removes a lot of times covers from his manga(tankōbon). I don't mean that he actually rips off the cover from the manga, but he removes the dust jacket(or, for me, the first layer) of the colored cover. Under this dust jacket, sometimes, there is another hidden not-colored cover page, that shows author's easter eggs.
I don't have pictures now, but, for example, it happens with some volumes of Monster Musume and Made in Abyss.
I didn't find anything about this on the web, and no one talks about it. So, I was asking: is there a term to describe this practice? Is it frequent to do it between collectors? I am just curious to know about this.
P.S. I know that it's difficult to understand, but I don't speak english natively. I'm sorry for bad english.
Edit: As Aki Tanaka said in comments, also One Piece sometimes has this kind of easter eggs.