Rabbit, to answer your post, I think that most guns of quality are designed to flex as little as possible with any load likely to be used in it. Yes, some flex is inevitable, but so little as to be negligible in causing wear. Yup, Rocketman, we don't know how many times these tight old bird guns have been fired, but we do know how many times, within a few tens of thousands, some of these old single trap guns have been fired. Virtually all of them made by Parker and those by Ithaca above serial number 400,000 are still tight with little or no maintenence. The flex factor must be very small in those guns.
My NID 4E #457465, from 1927, I seem to remember, is a heavy trap model, 8# 11oz. It has likely been shot somewhere between 100,000 and 250,000 rounds of trap loads. Probably never shot with heavy water fowl loads. Outside condition indicates very good maintenance. It is just about time for it to be put back on-face. They will shoot a long time under proper conditions, but not forever, even under optimal use. Open and close one without firing enough times and it will have to be put back on-face. My Parker "D" #2 frame, #107406, 7# 15 oz with 28" cut damascus bbls, has a seriously beat-on exterior, but shows little signs of being shot. It is tight on-face.
