My opinion on this: Like Wim, I think that Ndl could mean Niederlausitz rather than Netherlands.The latter would have been abbreviated in German as Ndld. rather than NL, as is common today. Even though the official abbreviation of Niederlausitz was N.L. (note the abbreviation marks, analogous to O.S. for Oberschlesien), there were many uncertainties in the spelling of geographical names. Example: You can very often see an Adolf Hitlerstraße on forms or photos, although even then Adolf-Hitler-Straße was the only correct spelling.
On the other hand, the stamp is a convincing argument. Obviously, there were abbreviation variants for the Netherlands as well. A few years later, rigid rules would have existed for such things.