Prosím o identifikaci nábojnice 20x138 B (solothurn long) pro FlaK 30/38 (rok výroby,země)

Hello everyone!
I recently got my hands on this cartridge. I believe it is a 20x138 B (solothurn long) cartridge intended for FlaK 30/38 anti-aircraft gun. While examining the casing I came across one peculiarity - the marking on the bottom of the casing. They don't match anything I've seen on these cartridges so far. There is no factory or test house mark, waffenamt, other symbols - just nothing. There are only numbers.

In the attached photo, the numbers are quite visible but just to be sure - left side 25, right side 51, top section of the bottom of the case 100. I am aware that these cartridges were not only produced in Germany but also in Italy, Greece, Poland, Switzerland, Finland, Hungary or Czechoslovakia (post-war production). The most similar to my specimen would be the Swiss one, but only in one number - 25. Otherwise there are other numbers and marks on the Swiss specimen that are not on mine. I'm guessing that the number 51 might indicate the year of manufacture (1951) but that's just a guess.

So I would be interested to know the origin of this cartridge (country) and the year of manufacture and if possible from the markings, also determine the type (HE, AP, APIT, HET, etc). Thank you in advance for the answer.