A report published Thursday in The Netherlands, says that MR1 is an early version of Android 5.1. That report noted that Android 2.0 MR1 later became Android 2.1, and that Honeycomb Android 3.0 MR1 later became Android 3.1. Continuing with the pattern, Android 4.0 MR1 became Android 4.1, and Android 4.3 MR1 became Android 4.4 KitKat.
According to a screenshot that was displayed on a large screen during the conference, Android 5.1 will not be a huge change from Android 5.0, and it will continue to be called Lollipop. Of course, by the time that Android 5.1 is pushed out to consumers, there could be some new features included in the update.
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source: AndroidWorld (translated) via AmongTech