I'd never marked my mags in any way but perhaps I'll start. I was at the range yesterday with a friend who had only ever had revolvers and wanted to try out my semi-autos. There were a couple times when he mixed the mags between my Ruger Security 9 and the M&P Compact 9. Those mags do look almost identical but won't interchange. I'm thinking maybe some marking on the side that's easy to see when the mags are laying on a table.