This is a genuine OEM sourced replacement part for Lawn and Garden equipment. A lock nut is a type of nut that resists loosening when properly threaded onto a bolt. It will have different applications based on your equipment. Please refer to diagrams for proper use. This item is metal an is sold individually.

You would need a wrench to tighten/loosen this nut.

This is a genuine Murray replacement part, it is sold individually.
The Jackshaft housing is connected to the drive shaft, which is connected to the pulley driven by the belts and engine.
Below the Jackshaft Housing, the blade adapter, blade, washers and blade nut are attached to the drive shaft.
The Jackshaft Housing does include the bearing and a zerk fitting.