that must have been a factory-original 1/2 ton then. they never made a 3/4 ton SWB back then. heck, they've NEVER made a SWB regular cab 3/4 ton except those super short trucks for airport tug trucks. i had a '78 similar to that...
ironic thing is, your "80" seems to have a '78 front end (round headlights, 78-79 grille) and my '78 had an '80 grille, lol...