Hi
I wanted to refresh myself Witcher 1 and I noticed that the install version from GOG I have backed up some years ago is:
setup_the_witcher_enhanced_edition_2.0.0.12.exe (exe plus 6 parts, 7 files in total )
whereas currently the non-galaxy download is named
setup_the_witcher_enhanced_edition_1.5_(a)_(10712).exe (exe plus 3 parts, 4 files in total)
which seems an earlier version.

Can anyone explain why the "old" version has higher number than the currently available on GOG? What is version 1.5 and what is 2.0?

Version 1.5 is the game exe version 1.5.0.1304.
the (a) just means it has galaxy support (I think)
The 2.0.0.12 was the old method where the installer version didn't reflect the game version.

The game itself may still be the same.
If you hold your mouse over the witcher.exe, what version does it say you have currently?