[MJN]

While I can't tell you why there are two different styles- neither of those shown are 3/4 or 1 ton versions- those have 6 spokes and measure 12" in diameter.
This site shows both styles you have - https://www.hubcaps.org/chevtruck.html

Heres a snip from the parts catalog; #6 is is the cap you're looking at and it shows several versions for Chevy only or GMC only and the part number changed throughout the years too.