Hello Stijn, are you able to upload a better magnified image of the marks. I zoomed your image up a little and there appears to be a "MB" in a rectangle. French for "white metal" unless someone else has any ideas? Then there is a lozenge mark that I can only see clearly as a "J & a cross" and then something else? All of which felt familiar to me. Have a look at the post below, the photos to which are missing regrettably.
viewtopic.php?f=18&t=51940&p=161863#p161863
Can you improve your photo?
Fishless