Its an eraser mask , something i used to use a long time ago in engineering drawing (before CAD, showing my age!)
Figured as it was only £1 i'd pick it up and give it a try as a scribing template.
It's pretty effective as it's flexible enough to bend around curved areas easily. It may not have the broader range of curves and circles that a verlinden template has for instance but as a cheap way to get into the technique you can't beat it!
Any decent hobby store or art supply shop would be bound to stock them.
