Last night, Microsoft released a new cumulative update for Windows Insiders with PCs in the Slow ring. KB4034450 brings the build number to 15063.448, which is a small increase from last week's 15063.447.
Last week's release was a bit of a mess. The firm published changelogs for updates to all versions of Windows 10; however, over a week later, many users still haven't seen the promised updates.
Today's Slow ring build only contains a single improvement, which fixes an issue that caused some laptops to boot with a black screen. If you're one of the users that didn't receive last week's update though, it will also contain those fixes and improvements.
We're only five days away from Patch Tuesday though, which means that Microsoft is going to be releasing new cumulative updates for everyone anyway. The reason that updates are getting pushed to the Slow ring is because the firm is testing a new way of pushing them prior to the release of the Fall Creators Update.