I think your guess of a massive over-pressure, like 70K PSI is very likely. From the fact that it broke the topstrap, as well as the cylinder, I was going to guess that the load was in excess of a 357 magnum load, which would be 35K PSI.
I would be interested to know the age of the revolver in question, since guns made after 1957, when the model numbers were added, are considered to be slightly stronger. But even older Smith revolvers are still pretty strong.
My guess of a very high pressure load is based on the history of the 38 special heavy duty, or 38-44 load. Before the 357 magnum was developed, Smith made 38 special revolvers using the larger 44 special frame that we would now call and N-frame. And heavy loads made just for these guns were called heavy duty or 38-44 loads. General estimates were that 38-44 rounds were loaded to about 28K PSI, and unlike 357 magnum, they would fit in any 38 special chamber.
It was strongly recommended not to put 38-44 loads in other revolvers, but of course people did try it, and the regular frame guns did not blow apart. The guns may have developed extra end shake, but they certainly did not break the topstrap as well as the cylinder.
So my guess is that to completely destroy the revolver, the round was well in excess of 28K PSI. And probably pretty far above that.