I can't attest to all the definitive differences between Gurneys and Westerns, but I can tell you that the set of Dan Gurneys that I have don't have the screw holes for the center caps, ...they are push in. They are also only 4 lug and have the valve stem lined up with the window as Mueller's link implies.
They also have 3 castings on the back of the wheel. One telling the size of the wheel, 14x5.5 in my case, one telling the maximum wheel load in lbs, and one saying either "Dan Gurney Inc" or "Dan Gurney Ltd"....or something like that...it's hard to read the last three letters.
In any event, the bore is roughly 3 and 1/4 inches in diameter, and I have 4 brand new center caps that fit the wheels I have. If your wheels have a similar bore, and you're interested, pm me, as I am not going to install the caps on my car. They are the "taller" caps and are roughly 4 inches long....pics included. There are no scratches on them, although it kinda looks like that in the first pic.
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