Another little bit of trivia for you - according to the Official Railway Equipment Register for April 1966, flat cars 6526 and 6527 were "...specially equipped with bulkheads for transportation of wallboard. Width of floor platform 8ft 2in.; width of end bulkheads 8ft 6 3/4in.; extreme width 9ft 2 1/2in.; extreme height 9ft 8in."
The 6500 series is shown in this issue as running # 6500-6544 with 43 cars in existance. The two missing would be the two listed above as they have their own separate listing.