400018-C: XM Separator Motor Assembly. Includes motor, mount, and timing pulley welded to the motor shaft. 400018-C is for those XM feeders wtih the gate motor mounted in the center of the gate assembly. For those with newer model XM feeders, see 400018 for side mounted gate motors.
The 400018-C is for early model XM feeders with the center mounted gate motor. The difference is the bracket is reversed in the C version versus the new standard version side mount.