I haven't looked at e-body seat back bolts in awhile but have been working with 68-69 b-body buckets and those bolts all have a steel washer with an integral rubber washer formed to it. They are about 3/16ths thick. The fiber washers go in between the seat back lever and the seat bottom upholstery for protection. The shorter bolt you have may be common to e-body if in fact they do not use the rubberized washers I speak of. I would venture to guess that the slightly longer bolts use the rubber washer and can be found on 68-69 A and B body buckets and probably others.