With permission from the highly skilled developer who generously took his time to create this build, I share with you Team Win Recovery Project version 126.96.36.199 for the MetroPCS Mega 6.3 (SGH-M819N). I can't take any credit for this build, but all I can say is don't forget to appreciate your developers who kindly take their time to bring things like this to you.
The need for this build arose after discovering the partition table on the MetroPCS Mega 6.3 is slightly different than on the international Mega 6.3, and it created an incompatibility with the more recent TWRP builds. If the AT&T variant ever gets it's bootloader cracked/unlocked (don't hold your breath), then this should also work on it too, as it's partition table is identical to the MetroPCS's. I have no idea about the Sprint variant.
Anyway, down to business...
I've attached the recoveries to this post in zip and tar format. If for some reason this isn't allowed, I've created alternate download mirrors. If you need this recovery in .img format, simply download the zip and extract it from that. Chances are, if you're here, you're already capable of handling that. Anyhow, enjoy...