Being entirely ignorant about Dzus I spent some time watching some Youtubes on how to install them and finally gain some insight that the ones I bought at the Oshkosh flea market are sized for a P-51 cowling, not what I need for my oil check door. I have never been comfortable with a frangible and losable screw there, and now I have learnt about retainer rings, and understand the sizes these things are available. I have just ordered a care package from ASS, and I'll need another for the Dzus. I have decided on AJW3-35 CAD WING HEAD STUD, with DZUS SPRINGS S3-225 and SR3SS Dzus Snap Ring. The aim is to install them for the oil check door and the nose weight compartment.
Trying to understand the concepts involved I assembled a test piece with my WWII versions. Washers were required to make the sizes work. Actually, these may be for a Liberty ship rather than a P-51...