This was covered in Napca years ago and again on this forum as well. Originally the concentric circles showed a mag to be 9mm and there was also a very slight difference in at least one dimensional aspect. This was proven by factory drawings. The Navy was the first to adopt the 9mm caliber and with the guns came the 9mm concentric mags. So the navy guns and Concentric became associated. But in the beginning it was for 9mm identification, not Navy identification. Earliest Commercial 4" Lugers were shipped with concentric mags marked 9mm as well. Because in those days the concentric meant 9mm not Navy.
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