Hi Bob, any Luger mag, no matter how it is marked, should / will work properly, in either a .30 Luger or 9mm healthy Luger pistol... I think the designation of either caliber on the bottom was only for protection against using already loaded and stored mags in possibly, the wrong Lugers, when both were present and had to be distributed?... and even that is a thin premise as there was only a short time, very early, that it would even be a possible issue?... A Mec-Gar current production mag is the best available mag for shooting Lugers that has ever been made... If your Luger doesn't work with Mec-Gar mag. the problem is probably not the magazine, but most likely your Luger....
.... if you want a magazine spring that exactly mimics the original spring, the GTHQ mag spring! Tom Heller, Luger Doc, has them in quantity... They are the best Luger magazine spring yet made.... Luger shooters on this forum mostly swear by them, and hopefully, not at them!...
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... best to you, til...lat'r...GT