There is a wide variety of magazine variations, more than P.38 pistol variations. P.38 is most of the time (but not always) stamped on the side of the housing and usually one or more inspection marks are stamped on the spine. However, there are numerous variations that differ from above description. Too many variations exist to describe them all here. For a good overview on the production process, development and variation of magazines I recommend the new book:
P.38 Magazines and Grips: A Collector's guide
from the Authors; D. de Vlieger, Ron Clarin and Wolf-Dietrich Roth
Also my magazine collection page gives a good overview of all the magazine variations.
In total four manufacturers of P.38 magazines are known.The following waffenambt acceptance stamps can be found on the mags (sometimes the mags have no stamps at all!).
-Mauser (E/135 early) (E/WaA135 late)
-Erste NordBohmische Metallwarenfabrik (E/706 and/or jvd)
It is also possible to determine the manufacturer by the form of the welding spots. Especially the ENM mags have quite characteristic welding spots. These are rectangular in contrast to the round welding spots of the other manufacturers.
Till the beginning of 1942 Walther provided magazines with serial numbers that corresponded to the pistols. After this date no more mags were stamped with serial numbers. No other manufacturers used serial numbers on their mags. Some wartime mags have serial numbers that were applied post war.
P38v and P38U are common markings on wartime magazines. The suffix letter v is an abbreviation for the German word "Verlangert", which means enlarged. The enlargement is only 2mm and is introduced as improvement. U stands for the German word "Ungehartet" and means Unhardened. Those markings were introduced around 1943 and 1944 respectively and can be found on magazines of different manufacturers.