340 K Member ??

You might have something here. In the second picture i posted on page one of this forum shows the rear of the k frame where the lower control arm goes into the frame. There are two flat pieces of steel welded to the frame next to the hole where the control arm slides in. I assume this was to strengthen the frame in that area for a heavier torsion bar. Or could it have been some type of an identification mark from the factory for a recall. Maybe early frames where cracking there and the factory was fixing them by welding steel around the lower control arm hole and identifying the fix with the washer because you cant see the fix once the control arms are back on the car.