I'm afraid I don't have any solid details as to the how or why, but...
They are unissued stamps printed in 1945 for the German Occupation of Laibach. Based on that date, one can presume that they might have been unissued because of the end of the German occupation.
Again, not sure if this is how these specific stamps got into the market, but at the end of the war, a lot of German occupation issues that had been printed – usually in small numbers for design or approval purposes – were "liberated" from the production, storage, or postal facilities where they were being kept. A lot of printers waste, etc., also made it into the collecting market as rare "varieties."
The fact that these are commonly found in perf and imperf varieties would seem to indicate they were "liberated" near the production end of the production/distribution process.
Here are the relevant Michel listings from the last English language version (2016) and the current German version (2023). Click to enlarge.


These issues were printed in single-color sheets with the different designs in rows. For this reason, they are commonly also found in vertical strips containing all the issues of a single color, with the full set being 7 (or 14 counting perf & imperf) vertical strips of 6.
