My guess is that its half an old sewing machine clutch drive from the power shaft era with a motor adapted to it. What seems odd are the two pulleys.
You're basically correct, it is the motor/clutch assembly from a power sewing machine, but probably originally individual motor drive.
The 'conical' part is the 'brake' which stops the machine when the spring-loaded lever operated by the foot treadle is let up. The clutch is operated by pressure against the friction wheel on the motor, allowing the operator to vary the machine speed somewhat by 'drifting' the clutch.
The pulley is for round belt, with two 'steps', or pulley diameters, for changing machine speed (a low gear, so to speak, for heavy canvas or leather, or 'high speed' for rapid garment production work)
cheers
Carla