Simple angle guides for knife and tool sharpening.
These are not designed to clamp to the knife or hold the angle for you. They are designed to help if you are learning to freehand sharpen. Just rest the knife against the guide, lock the angle with your wrist and start sharpening.
The groove at the top is there so you can wrap a rubber band around your stone and keep it in place. There are three versions with 15, 20 and 25 degrees.
The author hasn't provided the model origin yet.