Pretty sure nitro-cellulose was not used from 1969 (per CB750's) onward. No durability as can be seen in all the funky repaints that faded and washed off from fuel spills in the 1960's, 1970's, with or without clear, which was also lacquer. Easy to use, quick drying paint to shoot in the driveway, or under a tree!
I'd hunch we are seeing acrylic enamel on an original CB750, a much more durable finish the auto industry, and collision repair places were using then. Also, not seeing clearcoat over original candy red or candy blue CB750 K0 paint.
Far as I know, the only body work that Honda clear-coated early on were CB92 parts, front fender, tank and body side covers, which were all aluminum on early CB92s, also the first thousand or so CL72s..