When it comes to something like this the only real ways to tell are to compare side by side with a certified copy or to have it examined for certification by a leading institution like the Philatelic foundation.
Here's 2 more improved photos with some back light. I see the "5" is smudged a bit and top left perforation has small mark bit I'm really trying to see postage mark. There is no indication of any glue ever being on back...it's just clean paper with hint of a hinge. Sorry...I'm new to this and maybe my eyes aren't trained yet.