Given that the application was written in Delphi, I'd bet it's using some form of Partial Key Verification [0], which I wrote a fun blog post about a couple months back [1]. :)
Wow, that's super interesting reading! Thanks for the links! I will certainly be keeping this all in mind if I ever jump ship to proprietary software land ;)
The key validation algorithm in this software is extraordinarily simple, so I'm leaning away from there being anything fancy. I was unable to correlate keys used in later versions of the software with this algorithm, though, so you might be on to something. (I don't have a copy of a later version, but would love to check if I ever get my hands on one.)
Oh boy there are some interesting possibilities here with the partial key verification stuff.
What if you release a new version where, if the key is valid under the old check but not under the new check (indicating a keygen-ed licence), you start subtly screwing with the user. Like EarthBound or Spyro...
[0]: https://www.brandonstaggs.com/2007/07/26/implementing-a-part...
[1]: https://keygen.sh/blog/how-to-generate-license-keys-in-2021/