I confess I find this kind of stuff fascinating and worthy, I think, of its own thread. But I have no idea how to categorize it. Is there a name for this type of material and has any of it been cataloged? Would this be more appropriate on the US Covers forum?
There are many more to talk about, as seen in the excellent Stampsmarter database. The same form can have different date revisions, so a lot of items to hunt for here, and even new ones, yet to be discovered.