Back in November Sony released its first "Concept" Android build for the Xperia X, and it's received a couple of updates so far. These Concept builds are experimental and are meant to give people a way to test new software that's closer to stock Android than Sony's stable releases.
Today the company has started sending out another update for the Xperia X Concept firmware, and the big new thing is that it's now based on Android 7.1.1 and not 7.0. This makes Sony the first mobile device maker except Google to have rolled out the absolute latest version of the OS, even if we're talking about a very limited release here.
Sony's build number for the new firmware is 38.3.A.0.41, and the Android build number is NMF26F8. The update comes with the December security patches, which is a shame since the January ones are probably going out tomorrow. Anyway, there's a new Restart option in the Power menu, faster camera startup time, and support for PS4 remote-play (but with no DualShock support for now).
The Android 7.1.1 update is currently only available to those that have chosen the Experimental update track when they installed the Concept for Android release. That said, it's probably headed to the Stable track in a few weeks or so. As for when owners of the Xperia X who aren't part of the Concept testing will get Android 7.1.1, that's anyone's guess at this point.
Thanks for the tip, Dorian!