sean, if you look at the larger of that pair of sprockets - the one behind the small sprocket - you will see it has completely lost its teeth, every last one of them. The small sprocket in front is also badly worn, but the teeth have not fallen off yet. Both those sprockets have been run for years with worn-out chains, and the out-of-pitch chains have caused wear that widened the base distance between the teeth, undercutting the teeth.
