The following has come to my attention, where it certainly is a wrong number in the file name for at least two updates for Thronebreaker - The Witcher Tales.

(3-06-2020)
setup_thronebreaker_3553184_(64bit)_(38743).exe - (2726208 bytes) - 9c9b34a1c6aae4e9c808eba555a18910
setup_thronebreaker_3553184_(64bit)_(38743)-1.bin - (4292248062 bytes) - 68e1ee8c0739cfdf5e0e8ab33da8d562
setup_thronebreaker_3553184_(64bit)_(38743)-2.bin - (2241946652 bytes) - 28d0b220fd3aa8f134975b48347aee40
setup_4k_textures_dlc_3553184_(64bit)_(38743).exe - (513990056 bytes) - a0f5d6db918bb36f22720a2296028f89

(31-10-2024)
setup_thronebreaker_1.2_(64bit)_(38742).exe - (2723360 bytes) - 2c46ef0adf6b8faca1aa43cc0eeca7c9
setup_thronebreaker_1.2_(64bit)_(38742)-1.bin - (4292252158 bytes) - 1b075c9c61473ae5a62bab632b18fb8b
setup_thronebreaker_1.2_(64bit)_(38742)-2.bin - (2261482118 bytes) - e21231da5b3e88b86ff1b99d18d7058b
setup_4k_textures_dlc_1.2_(64bit)_(38742).exe - (513536488 bytes) - 4db581f2e8da80d988e5cfb108e2436c

(5-08-2025)
setup_thronebreaker_1.2_(64bit)_(38742).exe - (2721560 bytes) - b38eb45a5ceb42ffcedc2a1ab2c00c49
setup_thronebreaker_1.2_(64bit)_(38742)-1.bin - (4292254206 bytes) - 65975eb39c4cfb2057cb5360fa4832ae
setup_thronebreaker_1.2_(64bit)_(38742)-2.bin - (2255645304 bytes) - 5a949b9a46052305a36116b5f5dd3787
setup_4k_textures_dlc_1.2_(64bit)_(38742).exe - (514508088 bytes) - 71262e626e865e8825c7bef581f91844

NOTE
All the trailing MD5 values are different.

(31-10-2024) and (5-08-2025) have the same file names but also differ in bytes.

The older (3-06-2020) has a higher number in the file name by an increment of 1 ... 38743 versus 38742

So by my reckoning, the file name number for (31-10-2024) should be 38744 instead of 38742.
And the file name number for (5-08-2025) should be 38745 instead of 38742.

P.S. This is presuming I haven't missed any updates between those dates. Those dates being when I downloaded them.

And a similar thing for The Witcher - Enhanced Edition.

(8-11-2024)
setup_the_witcher_enhanced_edition_directors_cut_1.5_(cs)_gog_0.2_(77554).exe - 3d8467c76abe8cd16711ada268e1600f - (1294920 bytes)
setup_the_witcher_enhanced_edition_directors_cut_1.5_(cs)_gog_0.2_(77554)-1.bin - 149adafe38e79c246103e664613558c7 - (4293680638 bytes)
setup_the_witcher_enhanced_edition_directors_cut_1.5_(cs)_gog_0.2_(77554)-2.bin - baa94a6d83dc3a4703d8c414b4442ce5 - (4294967294 bytes)
setup_the_witcher_enhanced_edition_directors_cut_1.5_(cs)_gog_0.2_(77554)-3.bin - 5b74090cd3314dd0db1849c0dce5aa84 - (1589818064 bytes)

(5-08-2025)
setup_the_witcher_enhanced_edition_directors_cut_1.5_(cs)_gog_0.2_(77554).exe - 97cffb60d12f41bf03c84165a79a9157 - (1295600 bytes)
setup_the_witcher_enhanced_edition_directors_cut_1.5_(cs)_gog_0.2_(77554)-1.bin - 12107f584732052c4e11e09657df655a - (4293680126 bytes)
setup_the_witcher_enhanced_edition_directors_cut_1.5_(cs)_gog_0.2_(77554)-2.bin - ee77b189cf42bde0e89ea86c071b830e - (4294967294 bytes)
setup_the_witcher_enhanced_edition_directors_cut_1.5_(cs)_gog_0.2_(77554)-3.bin - b5fd3359b7129903a13baf731496f0c7 - (1592952147 bytes)

The (5-08-2025) files should have the number 77555 instead of 77554.

From what was reported in the updates thread, the recent updates for the Witcher games were only EULA changes, nothing in game files, so the version number would be the same.
For Thronebreaker it's weird, but they had renumbered 1.2 to 3553184 and then back again, may be different numbering according to language too.

Is this number meant to increment for installer-only changes?

EDIT: Sorry, I was busy looking up what changed. Thanks, Cavalary.

Many games have been affected by such a "decrement" update, all around the same time.

Updates not changing the build number are another thing, these have been happening for a long time already.

Your mistake here is to think the build number in GOG installer filenames has any kind of meaning. It does not.

The GOG build system uses a shared counter for the build version. It isn't incremented per game, but instead across all games, releases and updates. (The new Hexen + Heretic windows installer is build 83846.)

edit:
That said, I don't know either, why GOG sometimes decrements build numbers by 1. That makes no sense and breaks the usual convention of "bigger number = newer version".

Maybe it's a downgrade? A step back to a previous build?

It's not a mistake on my side, and yes I know GOG or DEVs can be inconsistent.

I have significant records to check via for the many GOG games I have, and generally the numbers I listed are build numbers and in the great majority of cases, increment. If they don't, then it seems pretty clear to me that someone stuffed up. Whether they are prepared to admit it or not is another thing.

It is true, that since GOG started pushing their Preservation Agenda, that more instances like the ones I listed have been occurring. Most of the discrepancies in the past have been huge differences in build numbers and file names making things confusing, but that is not the case here.

In any case, whoever is doing the versioning at GOG right now or now and then, have dropped the ball and it needs correcting, hence my thread. Maybe there is some Dyslexia thing going on. As you possibly recall, I've reported a similar thing before, but in this instance things are both the same and a little different.

I also don't understand, why they have to push a simple EULA change or adverts, without a file name change, as an update and notify us of such. They should just do the change and leave it as that, and not waste our time with unnecessary downloads.

GOG should care about their customers, and doing this sort of shit is just not right.

If they are going back to a previous update, then the MD5 value should match with that older file.

Any other kind of change, even if some of it is a downgrade, should still mean an incremented build number or something additional to indicate.

If they make an EULA change, they want the users to accept the new one, right? And while DRM-free means they can't enforce that, an update notification will make plenty download that new version and get the new EULA on installation.
Same for ads. They want people to see them, that's rather the point of ads, eh? And who knows, some may even be paid instead of mere promotion by GOG, and the number of downloads may influence how much.
Also, making such updates without marking them will lead to people seeing something change on GOGDB, or on their own for those who automate hash checks, or notice that their files have a different hash than those of someone else they know who downloaded them earlier despite having the same name, and then they'll complain about that.

What definitely should happen though is that all games should have changelogs reflecting all updates. Depends on the older file. Maybe it wasn't a previous public build. Maybe it's just something like noticing that the final build had a hiccup compared to the release candidate, or maybe just that the versioning was wrong in the final for whatever reason, and deciding to shove the RC, which wasn't public before, instead of it. Or if the build number is automatic, maybe based on date, maybe there are cases when it ends up one unit ahead because of time zone differences and when they realize it they dial one back to match what the devs have.
Don't know, just throwing guesses out there, but these seem plausible.

The version number as part of the filename is to indicate versioning of the game. Accompanying parts of the installation not part of the game aren't included in a number change. A repackage of an older game version might result in new non-game file included, but older game files inside. Hence a repackage may result in a different checksum than previous releases of the old game version.
So, you know, change a EULA text file and the hash changes, regardless of the game inside. Alter the compression level and other archive settings of the installer and the hash changes.

It's possible even updating the Galaxy dll might allow for a technical change to the installation without altering the actual game version. It depends on GOG's, or the developer's decision over what gets included in a version change, even a master or build one.