|This photo should show the correlation between the outer ring on the back of the lever selector plate and the flat spots.|
Here's a photo of the reverse side of the rubber washer/seal that shows that it's
almost identical to the front side, except it doesn't have the flat spots caused by the selector.
This seal is approximately 27 mm in diameter and the holes are approximately 8 mm.
Which leads me to think that a quick fix may be to simply flip this 4-hole washer/seal over, reassemble the selector lever plate and use the petcock.