Following in the footsteps of the HTC One (M8), he original HTC One from 2013 is now apparently in the process of getting updated to Android 4.4.3 KitKat.
This has started to happen in parts of Europe, with confirmed updates received in both Slovakia and Romania - in the latter case even for a carrier version of the smartphone, so not only units sold unlocked are part of the rollout.
As you may know, HTC has decided to incorporate the security fixes from Google's Android 4.4.4 release into its Android 4.4.3 update, so this will probably be the last Android version bump for HTC smartphones until Android L hits later this year.
The update for the original HTC One comes in at around 550 MB, and after applying it you'll be running Software number 6.09.401.5.
Aside from the new bits brought by the new version of Android itself, it also contains updates for some of HTC's apps, such as Camera, Gallery, and Sync Manager.
Thanks for sending this in, Ehsan and Adrian!