The image below is actually from an eBay listing for some homemade reprints, but they show what true mint condition pairs would look like.
Unnumbered, they were intended to be stuck on the back of the corresponding regular issue hockey
card. Kind of a neat idea. However, a card with a stamp affixed is technically considered "damaged." But it is not uncommon to find stars cards like Howe, Beliveau or Orr commanding premiums when they are accompanied by their stamp.
The stamps alone are more valuable when not detached, and when they have never been stuck to a card (or anything else). Many stamps you find will have been "unstuck" and show some wear and/or residue on the back. They are notoriously off-center, and even when in good shape can be found with perforation cuts running into the stamp. Add in that they are printed on thin paper stock, and you can spend a lot of time and dollars piecing together a complete set.
I got this PSA 6 Howe for a bit less than $25 including the shipping. I usually prefer raw cards, but this graded one was priced lower than most of the raw Howes I've seen online.