New Yamaha part number for an OEM jet pump reduction nozzle. It is not the propeller and is intended for specific Personal Watercraft (PWC) models from the late 1980s and early 1990s. 
Compatible Yamaha models
This pump nozzle is a direct fit for the following Yamaha PWC models: 
Specifications
Here are the specifications for the OEM jet pump reduction nozzle:
Brand New - Never Installed - Factory Yamaha 6K8-51318-00-9M OEM Pump Nozzle replacement part designed specifically for Yamaha watercraft models WR500 (1987-1993) and WJ500 (1987-1990), as well as custom Watercraft Jet Pumps, and possible updated or superseded Yamaha part numbers 6K8-51318....

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