My observations, consistent "across the board" with these models, having seen several original sandcasts, several original "K0's" and K1's: I've noted finding different appearing degrees of shine in black paint on the various factory black painted parts which does create dilemma and conundrum when restoring to like original appearance.
Some parts, such as seat pan, have a very thin coat of black paint and not very glossy; in fact, a sort of shiny dull, (not sure how to describe the apprearance) but sort of an "orange peel, "wavy" or "wrinkled" appearance from being such a thin coat. As i recall, same goes for seat latch, engine mounting plates, battery box, stop light switch bracket.
The frame is very definitely, at least in certain areas, very much a high gloss black with depth, much more so in exposed areas and less so in areas coverd by gas tank. Again, in those unexposed frame areas, the paint coat appears thinner. The lower triple clamp seems to always be significantly less shiny, but the paint is more thick than on the seat pan or battery box.
It's hard for me to imagine Honda used different sheens/shines of black yielding different grades of gloss appearance for these different parts. But, what do I know....
Seems hard to believe Honda would go to the time, trouble and expense using different black paints, considering the parts were headed to an assembly line for mass production.
Also, i have noted various black parts, such as seat pan, engine mounting plates, battery box, the paint tends to flake and appears no primer was under the black paint of these parts.
Relatively unmolested chassis 2157 is about as an original example as i have seen/owned and the above notes apply to 2157. I stand firm by my observations of black paint "types" as noted above. I can't imagine Honda would paint the frames and ancillary parts in anything less than in their standard gloss proprietary black.
And, it would seem not unreasonable, Honda would have had the same proprietary factory black paint finish, used on all parts calling for a black painted finish. From my observations, it does not seem difficult to imagine Honda applying more paint for a more shiny surface in exposed areas and less paint in unexposed areas, resulting in what appears to us to be different degrees of gloss, but is the result of barely one full coat of the same black color of paint, at least on some ofthe parts and on certain areas of of other larger parts.
What i can't understand, is the seeming lack of primer on some of the parts, as i have noted above. And, then, i could be wrong, but I will stick to what i'm saying until such can be demonstrated otherwise.
The other reason, i believe, the lower triple clamps are not a deep shine, even though the coat of paint is thicker, is the lower clamp is rough cast. Definitely there are varying qualities of black finishes on these parts.
If Honda actually did use a different black for ancillary parts and another black for parts such as frame, swing arm, i have nothing to go on other than my observations and assumptions.
When it comes to seats being produced in house or not, I would think Honda produced their own seats. (It probably is reasonable to say Honda did not manufacture and apply finishes to handle bar switch gear, gauges, front caliper, tire headlights, blinkers, chains, mirrors and etc.)
We also know from Bob Jameson, Honda manufactured their own majority of parts in different buildings and brought these parts to a staging area to go on to the assembly line. So, for example, seats could have been made in one building, wheels in another building and crank shafts in another. Some of the factory literature i have shows what i am trying to say.
Probably of significance, it seems a reasonable assumption, there were a number of men (and women) who did painting of black parts; some painted only frames and others painted ancillary parts. Some hung or set up parts to be painted, others took the painted parts down to a plce to finish the drying/curing period. Nor perhaps were all these different parts painted side by side or in a same common area. There may have been different booths for the different parts to be painted black. And a foreman culd tell the painters how paint was to be applied.
The only black painted part i can think of that is not deep shiny, is not like the black paint on the frame and other parts, is the upper triple clamp. The paint for the upper triple clamp is sort of dull, perhaps 70% - 80% gloss, i am guessing being dull to not cause sun reflection blinding to the rider. I have seen the flat uper triple clamp paint flake off in large and small flakes.
Hope this helps not cloud the issue more.
In the past 8 years since the club formed, we've learned so much and at the same time we find how much we have yet to learn.
We are a fine group of fellows, indeed and truly sre building a brotherhood, benefitting each and all.
Woud really be great if Vic World had the time to share his experience. There is likely no one more man the most knowledgable of all things sandcast, than Vic.