Polish P35 & German P35(p) pistol mags often have some form of script character, number or letter at the base of the spine. Pictured are three examples (L to R):
1. Polish "Eagle" w/script character(s) on spine & "D2 in oval" on floorplate.
2. Probably first half of first series with script character on the spine as only marking.
3. Late first series into the second, sub-contract manufacture with "boxed K" marking on spine and E | 189 WaA acceptance on floorplate. Also note the latter is retained by indents rather than the earlier pins.
One interesting observation is what looks like a partial "boxed K" stamp on the pre-war Eagle quite similar to the later German production example w/E | 189 acceptance. If the same stamp, this die (or multiples thereof) might have been put to use for several years. Polish pre-war pistols have perhaps more minute inspection stamps all over their components than any military pistol I can think of. Would concur w/krukster above that many of these were put to use as inspection stamps under German occupation production of the P35. There were undoubtedly a large quantity of Polish, partially completed magazines (the first) assembled under their auspices as well. Many likely had some degree of inspection even at that stage of production.