Nice one ,
this one is a German produced 20x138B AP-T round. Panzergranate Patrone L’spur. This particular one : Polte , Polte Magdeburg.
These were mainly made for export however some later ones are found with WaA marking & manufacturing code “aux” which is also Polte Magdeburg.
If I’m correct ... Holland , Finland , Switzerland , and a bunch of other nations used these German made AP-T rounds and were used in the 20mm Lahti M-39 (Finnish) anti-tank rifle if I recall correctly. I’m not sure if they were used in the 20mm Solothurn S18-1000 Anti-Tank rifle , but either way : 20mm German-made , Export round used in Anti-Tank rifles.
This one seems to be a very early one though , I don’t recall seeing one marked like this , most I’ve seen were ‘aux’ marked , later ones.
Only recall seeing this PM marking on the 20x105 L204 cases (Versuch)
Hope this information is useful & answers some of your questions , if anyone has any remarks on this subject , please feel free to let me know. Always open to learn.
Thank you