Update: Xperia Blog has confirmed this update is, in fact, Android 4.3. I can confirm that T-Mobile does, in fact, suck at changelogs. The version has also been tweaked to 10.4.C.0.814.
It might not be the latest and greatest anymore, but the Sony Xperia Z on T-Mobile is still getting some update love today. That mysterious Android 4.3 update that was pulled a few months back still isn't back, apparently. Instead, you get some small tweaks.