A couple of weeks ago, Microsoft released Windows 10 build 15042 to the Fast ring. The build removed the 'Windows 10 Insider Preview' watermark and showed no expiration date, implying that the Creators Update is close to completion. Two builds later, that watermark came back, but today, the version number has finally changed.
As you can see, it now shows version 1703 where previous builds still said 1607, which was the Anniversary Update. The same is true of Windows 10 Mobile, as you can see from the following image.
If you're not familiar with how Windows 10 versioning works, the first two digits are the year and the second two are the month, so 1703 would mean March, 2017. Of course, the Creators Update is slated to arrive in April - and PC will arrive before Mobile - but this shouldn't really come as a surprise; remember, the Anniversary Update was generally available in August, rather than July, as the version would indicate.
It's more likely meant to show when the final build will be compiled, which we're expecting to see this month. If you're a Windows Insider, be on the lookout for it, as RTM is really close now.
One other small change is in the watermark on the PC build. It's dropped the 'Insider Preview' words from it. This one is a bit more peculiar, as the entire watermark will be removed prior to general availability.