There's a good chance I'm going to feel pretty foolish when someone tells me what this little bracket is for on my 1976 Honda CB 400 F.
In the picture I erased distractions so you all would have an easier time seeing the part. (It's the red part....) My parts list(s) call it "STAY, SPEEDO UNIT". It seems to have been on all three (NA) years - '75, '76, & '77. It is part number 38555-377-000.
The two holes in the thing match the two holes for the horn mounting bracket. (Those two holes aren't shown in the parts list diagram, but they're just below the tank rubber mounting peg. The horn attaches from the other side of the frame.)
I just can't figure out what it's for. My best guess though, involves some sort of suspension rubber, kind of like the little stock suspension rubber which holds my winker unit (back by the fuse box). My guess is based on the way the end of the bracket is notched.