nitrolures
You may wish to look closer at the top left and the bottom left stamps. If you were to roll them into a tube with the face of the stamp on the outside, you might see small dimples on the back of the stamp that run parallel to the tube. If so they would most likely be the double paper verities. Just be careful not to crease of crimp the paper.
Ron

(158) 3c Green, Double Paper variety, HC, dark color, some SP's, Very Fine appearance