The Millen 74400 is described as:

'OCTAL, BASE AND SHIELD

Low loss phenolic base with octal socket plug and aluminum shield can.'

What you have is a 4 tube home brew device that plugs into the back of the
NC-300 using a Millen 74400 case.

If it has a couple of knobs, it could be a select-o-ject. If you unplug it
and lose audio, it probably is in the audio chain.

And, there is the possibility that someone cleaning up an estate found it
sitting on the shelf and the only place it could plug in was in the back of
the NC-300.

73, Colin K7FM