Mike - Your 6-clip rim (15 X 5 w/6 lugs) appears to have been available for 1947-50 1/2 ton trucks and 1946-48 passenger cars (same part number as shown in the extract from 1929-54 Chevy arts Book below). The other 15 X 5 - 6 lug rim with the 3 nubs appears to have been first used in 1961 on Light duty trucks from what I have been able to find in said year dealer handouts (see this
1961 Dealer handout). That dealer hand out shows pictures of the newer style over-wrapped hub cap that mounts on the external nubs and also indicates 15" diameter as standard wheel/tire size for the C10.
I didn't look further in that dealer handout resource to see how long those 3 nub wheels were used, but Kevin above mentioned they continued up into the late '60's. I think I recall seeing that 1970 wheels started using 4 nubs for hub cap mounting, but haven't researched to confirm. Perhaps others can add more details???
Anyhow, hope this is helpful.
