JB - I did a deeper dive, after seeing your pictures, into several Chevy resource documents and found that it looks like your 3 piece wheels could very well be original to the truck. They were apparently a heavy duty option for the 1961 (and could be other) model years for the C10. The first screenshot below is taken from the GM Heritage Site
1961 Chevrolet Truck Data PDF, showing there was a 3 piece 15" rim option for the C10.
The second screenshot is taken from a 1962 Chevy Master Parts Catalog that shows the passenger car and truck wheel options available with more descriptive detail. The 15" 3 piece rims are noted to have 6 hubcap clips and with riveted center section and the standard 6 lug wheel stud mounting pattern. The upper right drawing shows the 3 piece construction design.
6 clips where common for hub cap retention going a good ways back. I think IIRC, the 3 clip style came along with the Task force and later 1/2 tons, apparently with the exception of these 3 piece heavy duty wheels. Probably not alot of these wheels around as the vast majority of 1/2 tons came equipped with the standard one piece rims.
Hope this provides the details you seek.
