You were not to glue (attach) things to a postal card or post card. If that was done and found the card was to be re-rated to first class matter. I too own some of these tiny attached envelope post cards.
Notes on post cards also came in other covers
, in this case a hand cover:
The most common cards were the cards with a felt "flag" being glued on the post card. The flag carried a word, usually a town name. To avoid the additional charge the felt flags were replace by ink printed flags as part of the post card design. Below is one with both felt and just ink printed flags. Not caught for postage due.
Caught for postage due: