I very much doubt that the disk is original to the rifle - you can see that it has been placed in the disk socket recently. Clean edges, no patina, no infill at the side, etc.
Someone has just been out and bought a marked disk on eBay and put into a rifle.
Probably the rifle had a wooden plug in there or a blank disk.
p.s. see the other thread somewhere where a guy asks where he can buy a nice marked disk to put on his rifle....![]()