Chuck;
I would really not call the bbl wall thickness etc of the 21 as being "Over engineered". As to the underbolt as applied on the 21 it does absolutely nothing except latch the bbls to the frame. Properly fitted rotary bolts, Greener Cross bolts as well as doll's heads help absorb axial thrust at the top of the frame where they aid in resisting "Bending" at the juncture of bar & breech. No underbolt does this. The forward lug face of the rear lug on a dbl under bolted gun can (and does on many designs) hook into the frame cross member & assist the hinge pin in taking axial thrust, but still doesn't give the top connection. Virtually all well designed guns are designed with adequate frame strength to stand up to normal loads without the top connection. It should not be too hard to understand though as loads are increased above normal an efficient top connection will keep the frame from bending & finally cracking longer than the same frame without one.
8-bore, of course a tight fitting doll's head provides resistance against this frame bending. The fact that a Parker frame is amply strong for all normal use without it, has little bearing upon the fact that it would come into play in such a test as this. No doubt Winchester engineers were aware of this fact even if you don't seem to be. It is my understanding a Trojan was used for the test.
I still see absolutely nothing about it I would refer to as Over-Engineered, just a normally constructed gun made of modern materials.

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