It was an idea IIRC that never actually got put in. A headphone output isn't an easy thing to provide. One of the reasons is that headphones have a very hi-fi frequency response, rather than a guitar speaker's roll off above 5kHz. Therefore the sound will be harsh unless we add in speaker emulation circuitry. The next thing is that headphones typically have an impedance of 32 ohms, while the maximum impedance the output transformer of a tube amp can handle is 16 ohms. So things start to get very complicated, and I don't see this happening any time soon.
As far as I'm concerned the official spec is as listed on
viewtopic.php?f=16&t=2049