these markings will tell you , what sort of meter movement you have (moving iron/ moving coil. . ) , its basic sensitivity, if its AC or DC. And perhaps some other data.
Frank
I don't think there is a standard for meter markings but often they have a figure for the thickness of metal panel they are calibrated for. The ring of metal formed around the mounting hole has a small effect on the reading. Perhaps this is 1.5mm.