If you read the posts in this topic (which you didn't, I know that, since this has been posted already and it was proven wrong ) you noticed that Winver in very early days is wrong, then it was right for a long time until the full buildstrings came, sometimes it would show the complete buildstring other times just the buildnumber.

And for the NT builds since the early Whistler builds always have an buildtag for example: but is not always shown in the Winver.
That is why you need to check the registery for the actual buildnumber.

Even better, check the main files for NT themselfs, most of the time I use NTOSKRNL.EXE itsself.

(This last one goes for all NT's (NTOSKRNL), the registery with complete buildstring from pre-beta Whistler until the current builds).

@mrpijey:
Could you please complete your guide series and sticky them?
For all of them?

Sometimes identifying it can be hard, in the case of NT 3.1 Build 319-340 (and about Build 239, I wonder why on the kernel it is 1.0 while on the OS itself is 3.2??? Was it just random by Microsoft? for the 3.2 thing)

Press winkey+r and run 'dxdiag'. It will hopefully give full info

Just a heads up to anyone who's still not aware of this:

If you have an unknown build of Windows NT before version 4.0 and you need to make sure of the build number, but you can't actually install the build to check, do not use file versions of the kernel (NTOSKRNL.EXE/NTKRNLMP.EXE) as a reliable source of information, because they are not. The file versions of the kernel (and other files as well) have always been behind the actual build number reported in Winver until Windows NT 4.0 fixed this. This is the cause of many mislabels, such as NT 3.10.340 (mislabeled as both 319 and 328 based on the two kernel file versions).

The correct build number is actually hardcoded in the kernel. NT 3.51 introduced a kernel export called NtBuildNumber between builds 944 and 1024, which can be used to get the actual build number of the OS (along with the existing GetVersion calls). Before this, finding it is a bit trickier, but the simplest approach is to use a hex editor and search for the build number in little endian hexadecimal (it's usually stored next to some strings). Or you can disassemble the kernel and look for the code that displays the build number on the bootscreen and trace back the dword.

So, tl;dr: for NT 4.0 and later, you can stick to file versions etc. as they are correct. For NT 3.51 builds with NtBuildNumber, simply find and use that value. For NT 3.51 builds without NtBuildNumber and earlier, find the dword that stores the value.