Tumbling is a process by which solution is added to ribs, 8 to 12 percent is normal, (not the best ribs to buy).
I believe you mean Bronto ribs which are very large ribs, normally beef ribs.
The last one is 3 1/2 and down which is another indication of the size of the ribs. A rack would be 3 1/2pounds or less. Spares now days seem to be closer to 4 or 5 and down.
Jim