moderated Southern Railway 40 ft boxcars 505700-505848 (40-ft with 16' plug doors )

O Fenton Wells
 

Does anyone have information on these boxcars?  They were rebuilt 40 boxcars with 16' door openings and double plug doors.  Some had loaders insd=talled and were called container cars.  They are in the 1982 and 1979 ORER.
I would love to have a car diagram and information on when they were rebuilt and what cars they were rebuilt from.
Help?
Fenton Wells

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