Thanks for the brave response Thunderbox. The stamp was indeed very deep as quite a bit of metal has been ground off and you can still just see "something". I had thought it was a recent operation as the ground surfaces are still bright metal, but perhaps this gunmetal stays bright for years when exposed. I did not know of the "dimple" type proof marks. Is there anything around, book or photo library perhaps, detailing these marks both civilian and military as they seem seem to vary quite a bit. I have spoken to the proof house a few times and couldn't quite work out their procedures so I wonder if they might occasionally mix the marks on a single proof.