4 Seat belts in 69 C20?

[GTSideSwipe]

Hi everyone, I have a 69 C20 that appears to be completely stock. Original saddle floor mat, seat, matching engine block numbers and transmission. I do know it's been repainted once from a previous owner because there is paint on the seals and door panel chrome. What I'm wondering is if four seat belts were an option because so far I haven't seen another truck with four. The two middle belts are Robbins-JR-6704 belts and the outer ones are Irvin IC 4500.