Since no one has chimed in...
The crankshaft pulley *IS* the same part.
Does the 611’s balancer fail as often as the 5-cylinder 612/647s?
My understanding of its purpose is that as the different cylinders go through their combustion cycles, the crank winds up like a corkscrew the slightest amount, most pronounced with cylinder one. The rotational inertia of the pulley helps to reduce the amplitude of this twist, and the rubber mounting of the heavy ring helps further by soaking up the higher frequency vibrations that would otherwise bounce back and forth through the crank.
Over time, the heat builds up in the rubber and it can harden and crack and the ring severs its link to the crankshaft, which is left to undergo the loading and vibration of the combustion cycles unrestrained. Over time the vibrations can fatigue the crankshaft and lead to catastrophic failure (in theory... I have seen the math (which was elegant yet horrid!) but have only heard 2nd-hand stories of actual failures).
So, to head all that off, I have geared up to spend quality time replacing my failed balancer this weekend. Wish me luck!
-dave