I might be missing something here. I personally can't see any ground lugs in the photo. All of the common rivets are probably not current paths. Maybe a photo of the opposite side of the crossover board would help?

In the mod 19 there are four ground lugs. Each of the ground lugs is riveted to the board and has a wire soldered to it. (the current path is through the lugs) Connecting these four lugs together with a common wire eliminates any resistance due to oxidation between the ground lug and the board. You can also use deoxit as some have mentioned or you can physically tweak the lug to make a clean connection as long as it's not loosely connected to the board. Then measure for any resistance.