78SilveradoSWB and KIILew hit the nail on the head.
There were several changes in 1979 model year like the nice fuel doors, the introduction of the corporate 10 bolt front axle to replace the Dana 44, headlight trim change, change in sheetmetal stamping on the hoodnose, delay wipers, and a handful of other things.
1981 was another milestone year of changes for this body style trucks…some good…most not so good until fuel injection was introduced in 1987 then the yuppie GMT400 1988 model year truck came out in the fall of 1987 with 4x4 IFS and all that crap.....fuel for another thread.
