Back when I worked in the aerospace industry, we 'tumbled' all our stringer clips [aluminum] to break the sharp edges...It left a distinctive finish...The media was small triangular-rounded gray stones...IIRC, it was a dry tumble...
I can't describe the look of the finish because I don't know the words...And the 38 Special reloads I bought with that distinctive finish have all been shot off or otherwise disposed of...So I can't take a picture...
I'll have to try a lapidary supply house and see if they carry the stones/media...