Noel said
The "Spare Module Handbook" lists:
...
MX11-B
I apparently can't read. Should be "MX15-B":
https://gunkies.org/wiki/MX15-B_Memory_Multiplexer
Would that be part of this system at a Japanese computer museum? There is an 11/15 and the
11/05 has 'Unichannel15' on it:
https://www.kcg.ac.jp/museum/en/computer/mini_computers/dec.html