Pre-computers and Internet, we had to physically package stuff up and take it to the post office if we wanted to share something, so if you were going to go through all that much work, a lot of people would go ahead and include other things in the package, and the most common thing was, well I don't want to call them advertisements, but let's just say information about other bands, or zines, or labels etc. The little pieces of paper would vary in shape and size, and number anywhere between one and 12 or 15. Some were four or five inches square, and some weren't much bigger than a postage stamp. I found this to be more common with tapes from Europe, where I think the whole idea got started!
So, here's a couple of collage scans of snippets and bits that I used to get in the mail on a regular basis! Being a paper hoarder, I'm sure I have 100's or more of these things!