Ted;
Understand, I'm not doubting your word here as I have never even seen a Tobin. Just sort of perusing, do you have any ideas as to why these guns do not hold up. According to Researcher's post, these guns started to be built in 0904. Smokeless powder had already been on the market at least a decade prior to that so it didn't come about after the guns were built. The 1893 patent date would somewhat coincide with the general introduction of smokeless.
Most of my Lefevers have a last patent date on them of 1887 but they do not seem to have this problem of frame bending nor stretching. Over the years I have had a couple of the Baker Batavia line which does not have the "Draw Bolt" of the graded line. They too were tight & have only "One Function" bolting in the rib extension. They simply have an open-ended slot in a straight rib extension.
I will say though that unless those Tobin bolts were carefully fitted for their dual purpose I think that steep downward slope to the "Hole" in the extension was a great mistake. The slot in the Bakers rib has the bolting surface parallel to the bore axis.
