[John Sabato]

FACT: late manufactured models of the stainless steel lugers (regardless of the name brand on the receiver or toggle, were all made in the same production facility) were made to be used with a new magazine design with flat sides and no side seams. IIRC, they are not interchangeable with the earlier magazines which could be used in both stainless and original Lugers. The tube looks smooth on both sides much like the magazines for Colt 1911 type pistols.

FICTION?? ...I have heard two rumors over the years on why they changed the design... the first and most likely was for simplicity of manufacture... (Original design Luger mags ain't easy to make!)
...and the second story is that the manufacturer had planned to produce a .40 Cal S&W version of the pistol which would have required a different magazine design.