Nexus 6, Nexus 5 both receive latest version of the Android 5.1 Factory Image
Another reason to turn to the Factory Image would be a situation where an attempt to customize a Nexus device has gone horribly wrong. Flashing the latest Factory Image would return your device to stock Android. You can also use the Factory Image to update your Nexus 6 to the latest version of Android without having to wait for it to hit your handset.
In addition to the Nexus 6, the LMY47I version of Android 5.1 is also available as a Factory Image for the Nexus 5. That handset was built by LG and was the sequel to the LG produced Nexus 4.
source: Google via AndroidCentral