Wow, I worked at Spring Street Honda (Elmhurst, IL) in those days (1969, 1970), setting them up: I never even saw those instructions! Only the carb balancing was mysterious, though. The Honda reps told us to set the disc clearance at 0.004" to 0.006", and that was the sum total of the instructions we had.
Yes, you and I conversed about these instructions a while back.
I can see how the early docs might be scarce as Honda was shipping the bike almost before the documentation was ready, and early docs would get pitched as they were superseded, but I would think for later models every crate would have had a copy of the set-up instructions inside.
Granted, theoretically these would be single-use documents, not like a shop manual, but I mean apparently almost nobody stuck a copy inside the three-ring binder with the shop manual?
One could theorize that because the CB750 was so ubiquitous everyone learned how to assemble the bike so they never kept the instructions but by that logic CB350 instructions would be scare as well.
Oh well, I guess this has become a bit of an obsession for me.
I will keep searching for a reasonably priced copy and I will certainly share it here when I find it!