A new firmware for the oldest Xperia Z series including Xperia Z, ZL, ZR and Tablet Z appears to be certified on the PTCRB site. This firmware has the build number 10.5.1.A.0.283. Since Sony has been doing pretty much great with the software updates, there’s a strong chance for this latest firmware to be based on Android 4.4.4 KitKat.
Recently, Australian carrier Telstra also confirmed that Android 4.4.4 KitKat is all set to head towards Xperia Z, so the new leak actually makes a sense of this firmware to be Android 4.4.4 KitKat. However, as soon as the update rolls out we’ll be clear about it. We expect this update to come out in the coming weeks.