After 3 hours on the smoker, this is what we had on the top rack.
Some of those little pieces of trimmed belly meat were a bit crispy, but they were full of smoke and mighty tasty. Everything that wasn't crispy was coated in Dr Pepper BBQ sauce, wrapped in foil, and into a 250 degree oven for 1 hour. They were pulled and re-coated , re-wrapped, then back in the oven for a second hour. After resting for 30 minutes, slicing proved they were still not tender, so back they went for yet another hour.
These seem to have taken a lot longer than our last couple of batches and I think that is due to ribs of lower quality. Like Mr. Green likes to say, "Good stuff ain't cheap and cheap stuff ain't good". There were no grading mark so I assume they were Commercial or Standard grade ribs. We've been using Choice ribs the last few cooks. They were still a bit tough to slice but they had good flavor. My Mrs dubbed them "a little chewy" but I was so full from snacking on the trimmings I never actually had one. From the way they sliced I suspect she was spot on. We shall gnaw our way through these over the next few days and count it as another lesson learned in our great BBQ adventure. Even the failures are very edible.
Addendum: On warming some up a few days later I found the fears of toughness were not justified. The ribs were tender and moist with plenty of good smoke flavor. Plus, they left an almost perfect KCBS bite mark and the flavor was wonderful.