These are a multi-class product, useful for any slot car guide flags that have a 3/16″ diameter post (e.g. many 1:24 guides). Over the years, slot racers progressed from locking the guide in place with a collar, to using a washer and a screw, then to melting the screw into the guide, and finally started using #10-32 nuts to hold the guides into the cars. Each iteration provided more precise control of free play and easier adjutsments, but each had its drawbacks.
Back in the early 1980s, a talented engine machinist from what was then Southern Rhodesia (now Zimbabwe) emigrated to South Africa. His name was Harry Odendaal, and he was the first to produce this design of guide nut. They were much more accurately made than a regular nut, and the top-side drive meant they were much easier to assemble.
Available in two (2) versions - aluminum and brass. This is the brass version. Package contains one (1) Harry's Nut (brass).