> Because we can't ask Greg sadly, I think the Holmgren is the last around that would know definitively and I've personally lost track of him.

Steve Holmgren and the Arpanet Unix team are still around (at least they were 3 years ago). I just remembered that I put some of my notes & findings in a draft wiki that I wanted to develop for TUHS - but I never finished it:
http://chiselapp.com/user/pnr/repository/TUHS_wiki/wiki?name=early_networking

The recent find of CSRG report 3 and 4 may be the incentive I needed to complete my notes about 4.1a, 4.1c and 4.2BSD. However, still looking for the actual source tape to 4.1a - the closest I have is its derivative in 2.9BSD (https://minnie.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/net)