OK, I downloaded the official Watch package, including the public-facing font files, and they behave—interestingly, as well.Opening those font files from the Finder should (and does, in Yosemite) open a font preview window in Font Book. In 10.11, the SanFrancisco* files behave the same as the SFNS* ones in this case: Font Book completely ignores them. And again, going to File: Add Fonts allows you to select the files, and they do get copied into the right directory. The only difference is that they then become visible to applications, including Font Book, TextEdit, etc.My best guess is that SFNS* are tagged the same way LastResort.ttf is, to be internally available and displayable, but otherwise unavailable to the user.For something as critical as the standard interface font, this seems extremely cowardly and proprietary, and moreover totally a useless distinction.