Put another way, if you made a cube about 3/8" per side and set it on top of the breech end of the rib and mounted the gun you should see the bead just above the cube. The last joint of your ring finger would be that joint with the fingernail. In my case I estimate the third joint to be about 3/8" thickness when I place it over the rib.
In my first post I typed "see the bead just able it." I corrected it to "see the bead just above it."
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