After looking at the photo further, I am more convinced it is what is labeled a pulley on the schematic I found. If you look you can see a few inches from the end of the wood the diameter is slight less than the ends, a couple of wide leather drive belts would have done that over time. It would be heavy enough to serve as a flywheel to a lesser degree than a solid steel or iron would, but I will bet the bolts, end plates and shafts were purchased from a mill back east somewhere and the wood added in the local area.