Aha, one I can answer...

GTW, XAP, Auto, 572700-572799, 40'6", 6'0", 80000, 39
GTW, XAP, Auto, 572800-572999, 40'6", 6'0", 80000, 67

Both of these series are rebuilds from GTW's 575000-575999 series, all converted with single doors from the original double door arrangement. Cars originally built 1927, and rebuilt 1946. Although I don't have photos of these specific cars, the original 575000 series was a steel frame (single sheathed) design, so I would presume that this would also apply to the rebuilds.

Incidentally, my entries for the July 1950 ORER for the 572700 series show 59 cars.

Ian Cranstone
Osgoode, Ontario, Canada
lamontc@...
http://freightcars.nakina.net
http://siberians.nakina.net