Mike19652, first off, welcome to the forum
! Good to err on the side of caution before checking it out, but there's nothing odd or rare about this, either. We'd appreciate it if you would post an image showing this unfolded just as proof. And yeah, it appears the little corner margin piece is gone.
rogdcam explanation was a bit laconic for me.
I don't know how he could have been any more specific. Here's the third person who's posting that this has selvage folded over - it's that obvious
. Since this is a line perf stamp, the perfs on the left side wouldn't align like they do for this stamp. And who in the world would cut a partial rectangle out of stamp like this? Occam's/Ockham's Razor applies. You're starting with an assumption (that there is part of second stamp here) and trying to fill in the blanks, unsuccessfully.
Is there any documented data about what the selvage of this stamp looks like, and if so, do the measurements fit?
Still refusing to believe rogdcam and stallzer, who clearly know stamps better than you? Then someone might say that we are coming up with doctored images to prove what is being said here. So, you can find plenty of pane and corner block images online to check out for yourself for this issue. Measurements can vary quite a bit as slicing sheets into panes was not done to perfection by the Bureau of Engraving and Printing. Plus there is always trimming possible after the fact.