sys4.c: I think this is available in BBN V6:

In this file the only real network addition is Jack Haverty’s user timer variable, I think. What else is missing?

imp11a.c: available in two versions here:

impio.c: available here:

Maybe the compiler has a different opinion, but they don’t look obviously corrupted [??]

Also, the BBN-VAX files may be a solution. BBN sent four tapes with snap shots to CSRG in 1981. The TUHS website has the newest version, but there are 3 older ones (all 4 come from Kirk McKusick’s DVD set). They all have a slightly different set of files. These tapes also have IMP and ACC drivers in them, along with a token ring driver that was apparently based on Noel’s work back in the day. These files may also help filling in blank spots in PDP11 files.

Paul