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If you have an older, Kirk-era VS, there is a long, painful story about hubcaps and why some fit on some wheels and others don't.
Basically, when Kirk was forced to transition from original Mangel wheels to the Chinese copies, the old caps didn't fit the new wheels and vice versa.
I remember ordering some nipple caps from SoCal and having to return them because they were just way too tight to fit the Chinese wheels.
Caps that did fit the Chinese wheels eventually found their way into the supply stream, but it can be near impossible figuring out which wheel or which cap you have.
I eventually switched out the Chinese wheels for some old Mangels and had to get all new caps.
I've posted this before, but it may help illustrate the installation process Carey explained:
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