Mike--thanks for the excellent link.
According to Sloan, these cars were built BEFORE the 6500-series. They ARE based on the frame of the 100-series refers, and were weak, because the sills are the same size, hence the use of rail to 'beef them up.' They had the 4'-8" trucks as used on the short refers, another neat feature.
Sloan notes that the early 6500 flats also used the 4'-8" trucks, and only later were they changed to the cast steel trucks. As I model 1939, I think I will renumber my 6500 flat to an earlier number and change the trucks.
The C&TS does have a 6300-series car in the collection. For all the work cataloging cars for the NR and NHL, I only just picked up on this distinctive car!
Keith