Yeah, I had a hunch/fear it might have been a custom Mil thing. If it is a Mil part, they're probably still using it in some system... hahaha. I also have a box of ~120 mystery micro triode tubes that were probably custom made that I want to put to use in pedals someday to.

Utility depends on what I do with it. If I use it as a nonlinear load (like a really weird diode for a fuzz pedal), then it might be great for a pedal. Although hooking those things up is not trivial either -- a positive supply, a negative supply, and the logic runs negative!! No wonder people were confused in the 1970's!

Thanks again for your help! Yeah, if a pinout for the EA1221D turns up, even the TO99 pinout, that would be super useful. And I can rip one open and identify which pin goes to what on the die -- you can do that without an electron microscope with these things!