So in digging through some of my random parts recently I located a set of chrome axle nut covers that I bought years ago. These are listed for FXWG 80-81, FX 4spd 73-80, FL 4Spd 73-80, and XL 57-80.
Turns out the smaller one fits the steering stem nut on the CB750 (and likely other CBs) like a glove.