Are you sure that the marking isn't IMS? The I. M. Singer company made sewing machines, and many used round rubber belts (like big o-rings) for drive belts.
It looks like a drill press or like stated earlier pulley block. look at old drill press pictures and look how they were made it was a three piece cast iron tool. It consisted of motor, pulley, and drill looks like one part or like stated earlier same design could have been used on a sewing machine. The three positions would be speed adjustment.
Are you sure that the marking isn't IMS? The I. M. Singer company made sewing machines, and many used round rubber belts (like big o-rings) for drive belts.
Old sewing factories used an overhead shaft drive system (off a huge floor mounted motor on the end of that shaft with a big-*** belt that ran to each or all overhead shafts in an area) that used drop down belts to each machine that could be turned on or off with a spring loaded idler pulley at each station. Some times the belt adjustment was overhead. Some times at or near the machine. Early woodworking shops and wood mills used the same system.
Used to see it all the time here in the midwest in factories.
Could also have been part of a block and tackle lifting system depeding upon where it came from.
Decipher the makrings or stampings and you have a good chance of fining it's mfgr and thus use.