It is probably a 1920-era rebarrel. After WWI, Germany was prohibited from making pistols in the 9mm military caliber except under strict control and limits. But the country needed "hard" money desperately, so companies took the tons of Lugers available and rebarrelled them to 7.65x21, the original Luger caliber, for export to the U.S. and other countries. Collectors call them "the 1920 commericals".
Since the guns were refinished at the time of conversion, most are in quite good shape, with nice rust blue and "straw" color on the trigger, safety and ejector. There is some collector interest in those guns, but much less than in original guns.
The 7.65 Luger is a very nice cartridge; reliability is excellent due to the shape, and recoil is light. It is much used in Europe in countries where the 9mm is banned as a military caliber, and was the Swiss standard caliber until some years after WWII. Both SIG and Benelli have chambered guns for it, and Colt has made the lightweight Commander for it, also, for export.
Jim