I think I have a true 4.1 tape in my archives but ... I'm not sure it's on 3M (Scotch media). Those are in sealed 3M tape boxes in the basement 10 tapes to a box, and difficult to get too. They have been kept reasonable dry and mostly stable climate, which is why I keep them there not the attic. So far every tape, I have pulled from there we have succeeded in reading .. but I have not tried in a while because my triple density drive has load issues which I have not had the time to chase. I also do not have a tape oven [@Alan K . - I assume you made one. I'd love to hear your experiences with it].
As for BSD 4.1 kernel really was 4.0 plus a ton of small changes #ifdef FASTVAX [4.5 stuff is a little different than was reported the other day -- close but not quite]. This was the work wnj did to demonstrate that UNIX was within epsilon of VMS from a performance standpoint (i.e. the beginning of the UCB/Stanford war of what was going to be the supported system for Arpa). I'm 99.999% sure that the user level APIs are the same, the difference is he dropped into assembler in places, rewrote a number of internal routines etc. Basically, it tuned the c**p out of the 4.0 release to prove to DARPA that UNIX was just as fast or faster than VMS.
Note that the userspace code between the two released were different because time had marched on and more and more stuff was available and had been placed in /usr/ucb; plus more and more of the original v7 commands had been hacked/expanded. There really was not a lot of control of the userspace at this point and CSRG did not yet exist. As a for instance, the compilers and libraries had been hacked a great deal by a lot different people so even if the foo.c was the same chances are the /bin/foo was different binaries between the two systems [hey I wasn't a compiler person and I had hacked on libc to fix a stdio bug was causing my thesis to go in the toilet].
That said, 4.1BSD was the first really stable Vax code base and what a lot of people ran. It was formal release a lot of people outside of BSD had it both universities and commercial. There were copies at MIT, CMU, Standford, Harvard, much less some of the big public school likes Michigan, Wisconson, and Purdue. For instance, this is the kernel George Goble hacked on to create the Purdue Dual processor Vax. We had it a Tektronix, I know HP and IBM had it, and the original Marx brothers machines at AT&T Whippany ran it. Dale's folks in Columbus had and I think ihnp4 [Indian Hill New Products group] was a BSD 4.1 system. Plus, of course by the time I was at Masscomp, that was what they had had. [Sun did too, but I don't think they ever shipped anything with 4.1 BSD base. The original Sun OS was done for them by Asa Romberger's folks and was based on V7/System III. Joy had not come there yet].
As was reported the follow on to 4.1 BSD was to be supposed to 5.0 etc.... the naming stuff was described correctly in earlier email here. But all of that was >>post<< 4.1BSD. Post 4.1BSD shipping, CSRG had been formalized and now a project from the DARPA to support UNIX on new Vax hardware and to add extensions. As was described they ended up using a different naming scheme. Remember until that time, there was no formal CSRG project. It was like ever other University, a group of people hacking and swapping code changes with the reset of the Unix community.
So when the became a project and started to release things for DARPA is when formal tracking started. CSRG's Alpha's [or release candidates in today's terms] for these were called 4.1A, 4.1B, and 4.1C. 4.1A was pretty stable, but IIRC was not quite as radical is 4.1B (their's were signals got hacked and a number of new system calls added). 4.1B was not particularly stable and as Larry suggested 4.1C actually was usable and did not crash every day. IIRC The actual 4.2 BSD release took about a 9-12 months after 4.1C before Sam finally pushed it out the door [and I think wnj had left for Sun by then].
As Larry's comments about Masscomp, the original RTU 1.0 used a 4.1BSD kernel with a bunch of System III as the base (which really was an Alpha release for MSCP). It shipped to a handful of customers, but it was easy to crash (But we got it out the door and people loved it actually). The first version that actually was fairly stable was RTU 1.0A which was a mash-up of the earlier work using 4.1 plus tjt and I applying a lot of 4.1C to it (as I had brought 4.1C with me from UCB). RTU 1.1 or maybe 1.2 was when 4.2BSD was finally added. I created conditional symbolic links before that because we used them to create the Universe stuff and I've forgotten when that shipped.
Clem