Note that "Screwing the Barrels to the Action Flats" Doesn't really do anything to reinforce the bending of the breech to the bar. I would guess these Big Bore double rifles held up because of ample steel in the frame & had little to do with the method of bolting. Likewise the forward bolt of a double underbolt adds virtually nothing to the integrity of the bolting. The above mentioned Win 21 used only one underbolt with no top fastening. The one bolt was ample to keep the gun "Shut", the quality & quantity of steel in the frame was ample to take the load.
Of course a properly designed & fitted top fastener can increase the strength of breeching a great amount, as it gives the breech support both top & bottom.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra