BT shelf 4a
There is a common PSU module with a single mains power inlet, and a common display and control module for all the plug in cards. The rear panel on mine has a row of IDC header plugs for external connections.
It seems that there are a variety of NTU available, there are two fitted in mine, an NTU6B/2 which has a ribbon cable to 15 pin D socket attached, so I assume this is for X21 to 4w interface. The manufacturer of this card is not marked on the pcb.
The NTU11B seems a lower speed analogue NTU. This is marked as manufactured by Fujitsu.
Both of these share the same controller and display, this is selected by pressing the DISP button on the required card. From a BT document:-
There is only one LCD for a shelf of multiple NTUs. To select the NTU for which you want to display details on the LCD, press the appropriate DISP button. A yellow LA light confirms the NTU you’ve selected. The LCD will continue to show information from that NTU until you press another DISP button.