Found a 321 base here. But it has the tube sockets soldered to a printed-circuit board. Not the one you want.
When you say modified for different tubes, did they modify anything besides the heater connections on pin 1 and 12 of each socket?
If that's all they changed, this is easy, once you decide the type tube you want to use in it. Just those two pins, 1 and 12.
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