Cross slide half nuts for 7x12

[PixMan]

In looking at the photos it seems there should be a tab on the washer that would mate with the keyway in the screw. When I take it apart tomorrow I'll see if that's true and if it is I will take it out, put it behind BOTH of the nuts, lock it into the keyway, jam the two nuts together and mash one of the outer tabs into the wrench cuts on one of the nuts.

If there is no tab for the keyway, I'll remove it and go with set screws.