Its legit.I just saw on reddit someone claiming to have received the 5.1 OTA but with the Build ending in M, rather than D or E that the first two we've seen have been. Can't tell if it is BS or photoshopped though. I'll try and post a pic or link to the thread.
This is on the T-Mobile support site for the Nexus 6:
Beginning March 16, the Google Nexus 6 received a mandatory software update to Android version 5.1 / LMY47M available via Over-the-Air (OTA) and pull down over Wi-Fi only. This update will be available over cellular network at a later date and provides improvements:
New features: Device Anti-Theft,
Improvements: Software stability, Various bug fixes
I just read that also from here: Nexus 6 Devices With T-Mobile SIMs To Receive OTA Update To Android 5.1, Build LMY47M
I tried turning on WiFi and checking for updates but it still says I'm up to date. Guess I'll try again later.
I wonder what the difference is in our build.