Today, Microsoft released a new Windows 10 Insider Preview for PCs, bringing the build number to 17741. It doesn't contain much in the way of new features, but one thing that's changed is the version number. Redstone 5 is now officially Windows 10 version 1809.
Windows 10 version numbers are based on the date that the update will be released to the general public. The first two digits are the year, and the last two are the month. 1809 means September 2018, so that's when you can expect Insiders to receive the final build for the update. Of course, that doesn't mean that's when it will be publicly released. Non-Insiders will probably see the update in October.
While this doesn't really mean anything to the end user, it's a small milestone in the development cycle. After all, Redstone 5 has been developed under version 1803 for months.
It will be interesting to see what the next version of Windows 10 will be. Microsoft is ditching Redstone codenames in favor of those that reflect the date, similar to version numbers, but a bit more vague. The next update will be 19H1, meaning the first half of 2019. It seems likely that that will eventually go to the same naming scheme that we all know, and will be called 1903.
But for the time being, we finally have a version number for Redstone 5, which is 1809.