We all had our passions as kids, right? Where we would spend hours reading up on mysterious happenings behind closed doors, poring over grainy photos of a future in progress, and marvelling over what could have been and what ended up being?

I, too, had a special interest in the development process of Microsoft Windows, and one of the things that was a frankly inexplicable fascination for me was going through leaked development builds of Windows, seeing what changed in each revision, and even trying out some of them myself. One of the pieces of knowledge that’s been stuck in the back of my mind is the existence of Windows 7 build 6.1.7600.16384.

Here’s a quick anatomy of the build string:

• 6.1 is the version number, and it typically goes up with every new Windows version that drops. 6.1 is Windows 7.
• 7600 is the build number, and it is like what we might consider a release. The build number for a new version of Windows starts off at the final build for the previous Windows version, and goes up every now and then when they decide to compile an internal development copy of Windows to show to their QA teams and present to conferences and whatnot. Vista’s final release was build 6000, so when development on Windows 7 started, they started counting up from there, and the final release of Windows 7 was build 7600.
• 16384 is the revision number. If you make a mistake in a particular build, you can merge a quick fix and issue a new revision. This is typically bumped up artificially to 16384 for the final release that goes out to PC manufacturers and customers (and so this final build is often called Release To Manufacturing, or RTM).

Based on the above, it might seem to you that 6.1.7600.16384 was the RTM build for Windows 7. On 10th July, 2009, when someone hit “Compile” over at Microsoft, that’s what they thought it was going to be too. But it wasn’t—there were a couple of bugs someone found at the last minute, and so they had to run another build. 6.1.7600.16385 (hereafter affectionately called “Five”), built on 13th July, 2009, was the Windows 7 you know and love.

But, really, what’s the difference, right? They’re both the same kernel, the same build number, and one revision apart. The only change, allegedly, is a last-minute security fix in Internet Explorer 8¹. The rest of the operating system should be functionally (if not literally) identical!

For starters, I know, from the BetaWiki page for build 16384 (hereafter affectionately called “Four”), that it takes retail product keys (Windows 7 Beta and Release Candidate used beta product keys that wouldn’t work on a retail copy of Windows 7), it’s signed by Microsoft like an RTM build, it doesn’t have any dire warnings about being “Microsoft Confidential”. If you used it for a bit, you couldn’t tell it apart from Five!

The reason we know about Four’s existence is not just conjecture. A copy of Four was leaked onto the public internet, and was widely assumed by fellow nerds to be RTM. Fellow nerds were very surprised less than a week later when Five leaked and was subsequently crowned RTM instead. Some nerds had went ahead and installed Four, presumably to bask in the smugness of being the first people in the world to use the final release of Windows 7, and they were later found posting concernedly on the BetaArchive forum wondering if they could upgrade from Four to Five or if they’d have to do another fresh install.

That made me wonder—do you even have to upgrade from Four to Five? Could you not, just, like, keep using it? There’s nothing functionally wrong with it, assuming you don’t use Internet Explorer. And even if you do, presumably they’ll ship an update for it eventually and then there will actually be nothing to fret over. Windows 7 Service Pack 1 was 6.1.7601.17514, so presumably they can do updates across build numbers over Windows Update. So what would happen if you tried to run Windows Update on Four?

These old leaked builds are remarkably well-preserved for what is essentially leaked copyrighted material. You can just go and download Four off archive.org! It’s a bit hard to find because you can also just go and download Five off archive.org, and since it’s the actual retail release, there’s many more copies of that uploaded.

So I got myself a copy of Four, burned it to a USB drive, and installed it onto a computer I have lying around. It looks identical to Five, you can’t tell it apart unless you try really hard. You’d think it’s a matter of just looking at the build number in the “About [ProgramName]” dialog box, but that just says 6.1.7600—they didn’t start mentioning the revision number there until Windows 10. The only place I was able to find proof that someone hadn’t fooled me and the Internet Archive was the Internet Explorer version being 8.0.7600.16384 (the RTM one, predictably, is 8.0.7600.16385).

It doesn’t do Windows Updates. If you try to just go into Windows Update and click “Check for updates”, it’ll fail with error code 0x80072EFE—a common sight for a fresh Windows 7 install in 2025. This is because Microsoft released some updates that changed how updates were served for future Windows 7 releases, so you have to manually install three specific updates before you can use Windows Update as normal. Unfortunately, all three of these fail to install on Four because, and I quote, “This update is not applicable to your computer.”

I tried to install updates in various ways—running the .msu file I got from Microsoft’s website directly, manually extracting that file and applying it with the DISM command line utility (even with the /IgnoreCheck flag), and using the WSUS Offline Update tool. I also tried using Legacy Update, a project to keep Windows Updates possible and easy to access on old versions of Windows, and it tried the extremely bold strategy of installing Service Pack 1 before anything—which didn’t work, and left me with this error:

Fun fact: the only update I’ve successfully gotten to install on Four is the “System Update Readiness Tool” mentioned in the screenshot above, but after installing it there don’t seem to be any indications of it actually existing on the computer. It shows up as an installed update in Windows Update’s Update History page, but that’s about it. No shortcuts, no binaries, nothing. This is significantly more confusing than if no updates had worked.

But, yeah, even though Four and Five look, feel and perform indistinguishably, Microsoft knows what Four is. Four isn’t Windows 7, Four is just what could have been. I feel a little sad for Four, in that respect. But I hope it can take solace in knowing that it is my favorite build of Windows to ever exist.