I never waste things if I can help it. These castellated bits of paper are actually the off cuts from the ends of perforated sheets of paper. It’s where I have cut pages out of a sketchpad. One of the bits has black ink on it. The image was then mirrored four times in the Layout app I have on my phone. I don’t think I had glued these down, but I used some off cuts like this to turn into a block of flats (apartments) for a college project.