I've replaced some old GFCI receptacles in my kitchen, which live in metal electrical boxes. The old GFCIs did not have grounding conductors attached, but it seems that the boxes themselves are grounded. (I have tested for continuity between two of the boxes and I get a beep; I am assuming that the two boxes being electrically continuous is good evidence that they are both bonded to ground.)
I don't want to rely solely on the GFCI mounting screws for a ground connection and would like to connect a ground wire. The GFCI has a green ground screw, but the box does not have a green screw inside. How should I bond the wire to the box?
Here's a picture of the mid-run box. The other box is at the end of the run.