Here is a scan of an early 1915 Sperry & Hutchinson imperf block of 10. It was overprinted "AUGUST 1915 RED LETTER DAY" with some additional instructions below

According to an article by Roland Essig in the March 2013 Newsletter of the Wisconsin Federation of Stamp Clubs This was a promotional item and was to be pasted on the first page of a saver book.
The article also shows a similar totally imperf block from 1911, it was the only other one I could find.
I would include a link to the article but I'm not sure if I'm allowed to.