Robert,
If the round truly will not fit the -magazine-, it is likely that you tried a 7.65 x 25 round, such as made for the .30 Mauser or Tokarev. These cartirdges have an overall length of 34.7mm.
The .30 Luger cartridge (7.65 x 22) is shorter, with an overall cartridge length of 29.8mm. The longer cartridge simply will not go into the mag (or chamber, for that matter).
If you measure from the breech face the barrel should be closer to 3 3/4 inches.
As far as determining the date, it will be necessary to know all the markings on the gun--pictures of them will be useful. If the letter suffix is truly a it may be a bit of a puzzle.
--Dwight
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