I think they look like 68 sweptline belts... But the ones I have seen were black. The number is very close to the others I could find listed in a late 68 early 69 parts catalog.

2833037....Belt,seat,complete except A-100 and Tilt cab (This was black)
2833039....Van,wagon (this was black also)

If i had an earlier catalog (1968) It may show other numbers with different color options... I would guess the truck would have to be a "custom" to get the upgraded interior in a color other than black??