A: "It’s a urethane Sheave. The turned surface that looks threaded is to give grip to a urethane composite that is poured around it so a cable or belt can be driven by it or run across it. Source, I machined these for over 7 years. You would actually use broken or worn lathe inserts to do the OD so it was as rough as possible. 4 through holes give bolts enough pressure to stay on the shaft without walking off it."