I realize this is an old post, but I thought I would contribute something to the brown seat vs. black seat and color debate in regard to CB550Fs manufactured in 1975 and 1976.
I have three Honda sales brochures from that era pertaining to 550Fs. One is from 1975 and one is from 1976. The third is not entirely clear as to the date but in small print indicates “CB550F 1” and shows gauges with green faces, so I believe this pertains to the 1976 model year.
As to the 1975 brochure (featured with the CB750F) with reference to the seat, it says nothing about the color. It says only “New contoured seat for riding comfort.” The photos show a seat that is not definitively black or brown. Offered colors are “Metallic Blue and flake orange.” (The same colors are offered for the CB750F, which I think is incorrect.)
As to the 1976 brochure (featured with the 400F) it states “A comfortable seat-in brown, rather than black-finishes off the upper profile.” Offered colors are “Solid Orange and Flake Blue.” The photos again show a seat that does not look definitively black or brown. It does appear to be a different shade of color than the CB400F that it is pictured with. (P.S. I did not know that the CB400F has six gears!). It shows seemingly incorrect non-green gauge faces. Perhaps the CB400F didn’t have those(?)
The third brochure (presumably 1976) is not particularly helpful. This one, unlike the firs two, does not appear to be published by “American Honda.” Only the CB550F is featured. The seat is mentioned: “Contoured double-seat.” The color is not. The seat looks very black in the photos, but given considerations of color control and lighting, no definitive conclusion can be drawn IMHO. A rider with a brown jacket and black pants and boots is shown riding the motorcycle. The seat seems to match the black pants more closely than the brown jacket. Interestingly, offered colors are not mentioned at all. The motorcycle pictured is red-orange. The paint does not look metallic to my eye.
None of this seems to definitively swing the debate one way or another, but the explicit mention of the brown seat in one of the 1976 brochures at least suggests that this was something new in 1976.
I have a 1975 CB550F with a black seat, but given the condition of the seat, which is very good, I cannot vouch for its originality.