It was in a pile of parts that came with my 53 project

 

just a guess as it doesn't look factory, I d say it was installed on the rear for a step to get up into the bed

 

Looks like a vibration damper that was sometimes attached to the tail shaft housing of some specific transmission applications. I believe I've seen them on C4s.

 

Ray is right. I had an FMX trans once that had that same weight hanging off its tailshaft housing. It's not a necessary part for your truck.

 

Thanks. I never would have guessed that.

 

Yeah, early engine designs were made for cars that had few belt driven accessories and as such their mounting pads were cast pretty far back on the engine block. Later, as they began installing more and more options (p/s, a/c, thermactor pumps, bigger alternators, etc.) they needed to counterbalance the added weight by adding the tailshaft bobs.

 

No, Ray is correct. It's a static harmonic balancer. It absorbs and dissipates certain frequency harmonics at the opposite end of the engine. The rotating balancer is at the other end on the crankshaft. The need for these was overcome by engine design changes.