Miller,
The patent for the Tobin Simplex design had been around for a decade before Frank M. Tobin ended up with it, and started producing guns. You are astute in noticing that the bolting is comparable to other designs. But, the frame itself has had huge cuts taken out of both sides, and a large hole cut in the bottom for the lump from the barrels. My opinion, and it is free and worth at least double what it cost you here today, is the Tobin frame is compromised on resistance to twisting forces, and the design may actually aggravate them. The dual purpose use of the springs as cocking levers necessitated they be long, with a corresponding long action bar, to provide leverage to get the gun cocked. They also used rollers on the spring and hammer ends, clearly trying to reduce the effort it took to cock the gun. Early on, I figured out that the pin, part number 29, could be rotated, 180 degrees, presenting a fresh, unused surface to the barrel lump. On the guns I owned, this never changed a thing, they were still sloppy. The “wear” was actually a bend across the action bar, and Moses has accurately nailed the problem, on the illustrated gun.
That gun didn’t leave the Tobin factory looking like that.
The identical problem exists in far too many of them to discount that there is a problem in the design. Inspite of the fact that smokeless powders were available around the same time the guns were produced, many folks stuck with loading their own components with black, the same way they had for generations, and the problem may not have been well defined until the company was gone, or, nearly so.
I still believe it is a problem that develops with high pressure ammunition, and the guns would be satisfactory with loads more typical of black powder pressure. My 16 was mostly unused when I got it, with the lever well to right, as any new double gun should be. I had formed my “ low pressure only for Tobins” opinion by then, and used the gun with great success, and no issues. The 16 gauge Tobin frame was a bit narrower than the 12, otherwise they are identical. Perhaps the 16s held up better? But, it was a single gun out of many.
Of course, the Nitro Special that replaced it, has no such limitations, if you want to call them that way.

Best,
Ted