According to BuildFeed, which regularly posts about new builds of Windows 10, the first build of a new SKU for foldable devices has been compiled. It comes with the build string rs_shell_devices_foldables.190111-1800, and it's from the 19H1 development branch. The build number is 18313.1004.
The obvious conclusion to draw is that this is for Microsoft's rumored Andromeda device. While the project was shelved back in July, it was originally for a foldable PC that could fit in your pocket. It's likely now that it will be a larger device that's slated for later on this year.
Intel has also teased dual-screen devices in the past as well, so this idea isn't unheard of, and if it doesn't come from Microsoft's Surface team, it will probably come from a different OEM. Lenovo has played with the concept of a dual-screen convertible with its Yoga Book, the latest iteration of which has an E Ink display for a keyboard.
The Windows 10 device market continues to become more diverse, with new form factors, or different takes on existing form factors, becoming a regular thing. If this new shell for Windows 10 does make its debut with 19H1, the second half of this year should be pretty interesting in terms of hardware announcements.