Couple of possibilities for why there are two codes/packages.
The mfrs start their model year in the summer, getting the next year's cars in production for Sept/Oct introduction. But the legal standards for pollution measures begin January 1. My dad bought a 1968 Ford Country Squire wagon in late '67, which he got with no pollution controls. Beginning 1/1/68 they started putting on AIR pumps and the like.
One could be California emissions vs. regular emissions. That is unlikely due to the percentage of HD engines.
The last possibility is perhaps most likely. Through 1978, GVW above 6000 pounds required no catalytic converters, so the C-10 had them, but C-20, C-30, K-10, K-20, and K-30 did not. In 1979, this weight limit increased to 8500 pounds. Below that, cats were required, above it, no.
A surprising number of trucks prior to 1979 had 6100 GVW (like my 1978 K-10). After 1979, lots of trucks had GVW of 8600 pounds.
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