It's just a 3mm thickness disc with the picture on the top at 2mm. I use a 6 ton shop press to compress it with a 1" steel plate used to distribute the force. The print is a no name brand black PLA printed at 0.2 at the fast setting on an AnkerMake M5. You can also see the residual ink, the stamping only takes a small amount. I did a few at one time without having to reapply.
It ended up being solid just because of the standard wall thicknesses, but PLA prints are quite strong from a compressive standpoint. That said, I am sure I am not applying the entire 6 tons. I have done similar experiments with a bench vice with similar results too, it's just a little harder to line everything up give the jaw depth.