Differences did exist, otherwise DWM wouldn't have bothered marking the magazines. According to some old world experts the following differences may be observed:
-Difference in spring strength between 9mm and 7,65 mags.
-Altered magazine knobs to show the caliber (the 'navy pattern').
It seems that these modifications were rather short lived, and around 1906 - 1908 the interchangeability was improved, so that separate marking of magazines was no longer required.
In the 1970s, Mauser also produced separate 7,65 and 9mm magazines (different spring strenghts) for a while.
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