I also have a stash of these, via Brent Jesse - his description:
“This is a nice industrial type which is nearly identical to the standard 12AU7. RCA made a nice blackplate version of this tube, Sylvania has a gold pin version, and GE has it in their 5-star broadcast line. The plate voltage rating is a little lower than for a 12AU7, but for most applications, it will work fine. It has a rugged cathode and should be long lived, similar to the 5814.”
That specially-designed cathode is said to withstand long periods of time in cut-off without damaging the tube (cathode poisoning). This made them a great choice for NORAD computers, which led to a good supply of them here on the West Coast.
A great article on using computer tubes like the 5963 for audio use:
I use them in my TADAC (Tube Audio Designs DAC) and have, like yourself, marveled at the quality and value. They are dead-silent and seem to have an unusually long lifespan. Very satisfied with the sound they produce.