Did a little more research and the only pinstriping available in '79 was on trucks that had the RPO Z77 Sport Option. It was listed as "Multi-toned decal striping on front fenders, cab and pickup body sides, hood and tailgate, with Chevy Sport lettering over rear wheel openings." In other words, not what you have on yours.
I used to work at an Oldsmobile dealership and it was common for them to take low optioned vehicles that weren't moving and add some pinstripes, custom wheels, or a vinyl top to dress them up some. Your Custom Deluxe probably got a similar treatment when it was new.