After reporting about the Nexus 4 and Nexus 6 and Nexus 5 running Android 5.1, today the Chromium issue tracker reveals a Nexus 7v1 running Android 5.1. The 2012 version of the Nexus 7 tablet runs the two week old build LMY47D. Android 5.1 is expected to be released soon and should bring back silent mode, improve system stability and RAM management, fix bugs and come with some material design changes.
The LMY47E build is about two weeks old, starting with Cupcake, individual builds are identified with a short build code like LMY47E that give an indication of the build date. The first letter is the code name of the release family, e.g. L is Lollipop. The second letter is a branch code that allows Google to identify the exact code branch that the build was made from.The next letter and two digits (Y47) are a date code. The letter counts quarters, with A being Q1 2009. Therefore, Y is Q1 2015. The two digits count days within the quarter, so 47 is February 16th 2015. Finally, the last letter (D) identifies individual versions related to the same date code, sequentially starting with A. So D is the fourth version of that day.
Google states on their website that the date code is not guaranteed to be the exact date at which a build was made, and it is common that minor variations added to an existing build re-use the same date code as that existing build.