OH yeah, in the 30s the postmaster general got into some trouble. He gave out some sheets of uncut/unperforated (imperforate) of the National Parks series stamps with no glue to people for gifts. And this lead to a bunch of souvenir sheets being printed and I think even releasing imperforate sheets to the public to counter the rarity.
Let me see if I can find an article.
FYI that series is gorgeous. I love old school engraving on stamps.
