To teawithpuppits-It's true. They do exist! I got lucky for the first time and got a couple. Being self adhesives, not like regular gummed stamps as Kuhil mentioned; they should have stickey gum on back of stamp. Because of the error they are printed on the backing paper which has a shiny surface, therefore "ungummed stamps"
. The gum is still on the stamp paper which is now the cover. So the paper was fed in upside down during printing. They were correctly die cut. What ever you do, don't lift the stamp to check for glossy back. Just get out that trusted longwave UV light and look at the item. Normal cover will"flouresce
while the stamps will appear dull, and so the "error booklet"
cover will appear dull and the stamps will flouresce. Very popular commerative "Queen" stamp booklet compared to definitives. I have not heard or seen any others mentioned anywhere else or for sale. See attached photos to compare. Have fun searching as I went to 20 P.O. and only found them in 1 SDM location. Good luck hunting.