Al is right. Tom West led the Eagle project in Westboro, MA, which is documented in the Soul of the New Machine. The Eagle project became the 32-bit MV/8000 Eclipse [see https://en.wikipedia.org/wiki/Data_General for more details].   I knew a few of the HW guys, as I went to CMU with one of them, and a couple of them came to do the Stellar CPU a few years later. But I did not know the SW folks there like I know the DEC folks, particularly since I never worked there.

That said, WRT to their later UNIX box, I did have access to the same at Locus.  As we did a lot of work for DG, adding features and helping them with POSIX/FIPS and SPEC1170 conformance — IIRC, we added the FS Switch for NFS support.  It was probably the easiest UNIX kernel I have ever worked with, with the shortest learning curve. I may remember that the User API was based on SVR3/SVID, which means >>IIRC<<; it was based on Streams/TLI, but ISTR is a user mode sockets layer for porting (but I'm not sure of that -- it has been almost 30 years now).    The kernel itself was a scratch rewrite with SMP in mind. The locking scheme was clean and simple and worked well to order 32/64 processors in our tests.  Moreover, the kernel was well-documented. I do not know for sure, but I remember being told by some of the DG folks in NC that it was originally planned for the failed Fountainhead system, which was canceled after West and the MA-based folks delivered Eagle before the new system came from DG NC.

Also, I believe that DG had in response to BLISS a low-level system language they called PL/N, but I don't know much about it/I never saw it - I'm told that it was a >>very<< subset PL/1 syntax, but like PL/360 - exposed a lot of hardware.  ISTR that they developed some MV series compilers with it, but since Eagle was a 32-bit super set of Nova, AOS was ported.  FWIW:. Since Multics had used PL/1 and supposedly Fountainhead was heavily influenced by Multics (as was Pr1me, by the way), it would not surprise me that PL/n has a PL/1 'flavor.'

I'd love to know for sure >>but my WAG<< is that that core work for Fountainhead was reimplemented in C for their SMP 88000 box, and they added a UNIX API layer to it.