It is important to note that when the guns were built new, the rib extension was not on the barrels when they were fitted to the frame.
The initial barrel fit was done without them. So fitting was far less involved without that in the equation.
After the flat breech was fitted to the frame, the rib extention was attached to the barrels and fitted to the frame. Then the rib concave was final profiled along with the frame and top lever. Then matted.
You will note evidence of this in TPS where they name that Rib extension fitting was a separate operation in the factory.
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B. Dudley
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