My old Briggs data for Model 28's shows a 16 pound crankshaft as a part number and two styles of cranks that weigh about half of that. The heavy doesn't have a syncro-balancer and the other two are the removable and non removable eccentrics with syncro-balance. What year? I have no idea. I too have never seen one of the heavy ones.
Check this out;
http://en.wikipedia.org/wiki/Briggs_%26_StrattonThe Synchro-Balanced Engine - This 1966 innovation was designed to attenuate vibration caused by the reciprocating mass of a single-cylinder engine. The design was a series of counterweights opposing the crankpin.[10]
10. ^ The Legend of Briggs & Stratton by Jeffrey L. Rodengen, Chapter 11, pages 121-122