http://www.shootinguk.co.uk/reviews/shotgun/drilling-16b8mm

"A further complication is proof marks because those proved during the years of the Third Reich are not accepted as valid. This means it would need to be reproofed prior to sale with the associated cost and the possibility of failure.
Read more at http://www.shootinguk.co.uk/reviews/shotgun/drilling-16b8mm#wbxR1LdLTMSjZqzO.99"

realy?
there is no valid reason other than being stupid, its a f......g proofmark and this whole proof thing was than making better than before