As above, my long dead grandmother when I was a little boy was a seamstress. She used this tool to cut button holes. The blade slides into the tool. All the way in would be at 0. No hole. Slide the blade to the next no. and push it into the cloth and that's the size hole you would have. As the blade slides forward and the slide lines up with the bigger numbers, that's the size hole you will cut.
This is a real fuzzy memory as I was only 5 or 6 years old. I don't even know if this was the original purpose or if its just what she used it for. Also she used the scalloped sides for pinning cloth. Run the pin between the scallopes from one side to the other and it keeps the cloth flat while you pin it.
