Why have not heard of this before? US and Euro hulls are not the same exactly, there are two main differences. Euro hulls are most often straight walled with oversized primer pockets that can cause issues if reloading with American primers.
There are also American hulls that have been reloaded using Euro primers, stretching out the pocket.
This little tool from BPI uniforms the pocket from the OUTSIDE. It’s a small die that you put over the pocket from inside the hull, and tap it a couple times with a hammer. Viola! hull can use American 209 primers again. It is basically a resizing die.