In the log Don't go full Numitron ! Unless... OK whatever. I examine how to reuse the techniques developed in  #Numitron Hexadecimal display module  and apply them to decode all the fields of the Y8 instruction.

For the register name I came up with this diagram :

This decodes to the following symbols :

The segments E1 and G1 are always on (for A, d, r and P) so I modified the diagram a bit, later.

I didn't optimise the replicated 1 and 2 because it would make the relay side too complex for little gain (I only have to build 2 decoders). Here is the result :

To test the circuit, I have built a test device with a "standardised" 26-pins connector and 3 Numitrons. To inject data, I will reuse the connector and pinout of the existing Numitron hexadecimal modules.

Note : the module can be powered down by disconnecting the +3.3V power line. The diodes will prevent shadow segments anyway, even if another decoder is connected in parallel to the Numitron (for example to share the Imm4 and Reg formats).