Running medal

This Birmingham 1939 fob were given out by E.P & Border High School Athletics Ass. for the 100yds in the same year. This one I like because it were made in beginning of WW2 and must have been shipped the same year to South Africa. The medal seems to have a mat/pitted surface, Would it have intentionally be put one and how? The sides and engraving are still shinny but the hallmarks also got this mat texture.

Run marks.jpg
Run back.jpg
Run front.jpg

I’m not an expert in silver work but I think that the pitted finish is a coating applied after hallmarking - similar to the coating seen on the inside of cigarette cases. I don’t know what its purpose is; it may have anti-tarnish properties. I’ve tried searching the net before for information on this but have failed so if anyone out there can add any more I would be interested to know.

Almost seems like it has been sandblasted. I have two Birmingham medals with this finish. May have been etched by nitric acid.