Although it is not a perfect match to the Yokohama cancels use on mail carried on the July 1, 1869 PMSS trip from Yokohama to San Francisco, it is probably a genuine Yokohama cancel from a piece of mail carried on that trip. It is quite similar to the cover in Figure 7-21 in the book "United States Post Offices in China and Japan, 1867 to 1874."
I am not sure what the Philatelic Foundation would say but it is my opinion that it is a variant, late state of cancel type "YOK-Can4".
Thanks so much, Richard, for your opinion and for the reference to your book. You may see an order for a copy coming from me soon! I knew nothing about the US post offices in Japan until my interest was piqued by several 1869 pictorials in Siegel's upcoming sale of the Edward Morton collection. I noticed a certain similarity of the cancel on my #116 to the "X" cancel from Nagasaki, and I thought maybe mine could be from another Japanese city.