The com.apple.MobileAsset.MobileAccessoryUpdate.A1523.EA
MobileAsset type contains firmware updates for AirPods (1st generation).
It's published in mesu for iOS (manifest) and macOS (manifest).
The manifest was first published by 1 February 2017 , when the first firmware update was released.
For the release history of AirPods (1st generation) firmware updates, see the OTA Updates/AirPods (1st generation) page.