This Is the 5/16 x 24mm Lite Lock from Harry B. Higley.
An electric starter or a backfire can cause the propeller to spin off
especially with four-stroke engines. Any mechanic knows a second
nut, screwed against the original one, locks both tightly in place.
This Prop Lock replaces the propeller washer. A normal nut, or Heavy
Hub is screwed against the Prop Lock.