DVZ,
Those are 19.5" wheels like used on a P30 chassis and 3500HD trucks. They have a 5.25" center hole and 10 lugs on a 7.25" circle, fit over 5/8" studs, can work over smaller ones too.

Check out the tech tip mentioned above.
Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-