This ad comes from The Dragon, issue #35, published March 1980.
It's an ad for fantasy bookplates.
Let me back up. In the 80s, there were these things called "books," which were like e-books, but more papery. And each book contained only one manuscript. And this wasn't because of some harsh DRM scheme; the technology allowed for only one story per book. I think it was a RAM issue or something.
Anyway when you had a particularly nice hardcover book like the Player's Handbook, you might want to mark it to discourage other people from stealing it from you. I did this with crayons, but fancier people might employ a "bookplate," which was a nazzy label affixed to the inside cover of a book. Often you would see bookplates used by libraries to identify their property.
Oh, so there were these places full of books called "libraries" that you could enter FOR FREE and read any book there FOR FREE, just like a Borders or Barnes and Nobles but without the coffee. And unlike Borders or B&N, you could borrow the book FOR FREE. Can you imagine how primitive that was? Where was the free market in this scheme?
So this dude or lady made some bookplates and took an ad out in The Dragon, whose readers were likely to have hardcover books. You see the business opportunity already, don't you? It's call market research.
Now, there wouldn't be web sites for another 15 years or so. That means you couldn't just download the PDF version of the bookplate, printed it on your Star Trek lazer printer and tape it to your DMG. No, you had to write for the catalog. Write a letter! Like a damn monkey! And then, you'd have to wait days, maybe WEEKS for the thing to show up in the mail. Only then could you make your selection of bookplate and write ANOTHER letter BY HAND to get the actual thing you wanted in 4 to 6 WEEKS.
Can you see why there was recession after recession in the eighties?
You kids have no idea how good you have it now.