Pointers to more information:

@NCommander (Michael Casadevall) made an attempt to get the code compiled and running:
https://www.tuhs.org/pipermail/tuhs/2022-October/026522.html
https://github.com/NCommander/network-unix-v6
https://www.youtube.com/watch?v=BWrZIc1rG5s
https://www.youtube.com/watch?v=Pna_0iBE0D4
https://www.youtube.com/watch?v=auts_uJCX1E

There are several IMP interface drivers:
https://github.com/pdp11/network-unix-v6/tree/master/nosc-files/ncpk/drivers
I.e. IMP11A, ACC LH/DH, Illinois?, VDH.

IMP11A is a DEC device, from CSS "Computer Special Systems"
https://gunkies.org/wiki/IMP11-A_ARPANET_interface
https://www.bitsavers.org/pdf/dec/unibus/IMP11-A/IMP11-A_PDP-11_Host_to_IMP_Interface_Feb1975.pdf

ACC LH/DH is by Advanced Computer Communications:
https://gunkies.org/wiki/LH-DH/11_Local/Distant_Host_Controller

VDH is the "Very Distant Host" interface, essentially a modem connection.
https://www.rfc-editor.org/rfc/rfc263.html
https://gunkies.org/wiki/VDH/11_Very_Distant_Host_Controller