Proofmarks
Most countries proof their firearms. When the arm passes the test a proofmark is punched on vital parts (Duits: Beschußzeichen, French: Poinçon). On this page you'll see some Belgian proofmarks. The Belgian proofing institute is situated in Luik (Banc d'épreuve des armes à feu). Due to a fire in 1922 all the archives of inspectors were lost resulting in the fact that punches applied before 1922 can't be traced back to the applying inspector or the date the arm was punched. More proofmarks: http://www.proefbankstempels.nl/