He's trying to determine whether the belts are nylon or not. I doubt they're cotton as they'd stretch too much in an accident. Probably nylon since that seems/seemed to be the yarn of choice back in the day for seat belts. Now they make racing restraints out of pure polyester since poly doesn't have as much stretch as nylon does.
If they're nylon he'll send them to me and I can dye them deep black for him. If they're poly then they won't be able to be dyed the deep black he wants because poly has a very narrow range when it comes to dyeing deep colors with an atmospheric machine. In those instances they have to dye the yarn under pressure so that the dye penetrates into the synthetic fiber and then weave the belt with the pressure dyed yarn. If they're mixed fibers like a nylon/poly......then we're going to have a problem as it will dye a charcoal since the poly won't take the dye but the nylon will.