[AHorsa]

Thanks for your reply. It is not a marking, it is a corrosion pit and a part of metal layer being undermined and liftet by corrosion as far as I can determine

A remark: on the third last image it looks like the crossguard is made from two pieces. What looks like a welding trace is just wax. The crossguard is made from one piece. The whole for the tang is fitted with two triangular pieces of steel inserted on both sides (see second last image)