Peter,

A partial answer.  According to my July 1958 ORER:

SLSX 71000 - 71999 - double deck stock cars (40' length, 8' 6" width, 8' 7" height) - 344 in service
SLSX 3100 - 38899 - single deck stock cars (40' length, 8' 5" width, 8' 7" height) - 55 in service
SLSX 3900 - 3999 - single deck stock cars (36' length, 8' 5" width, 8' 7" height) - 34 in service

I could not locate the reporting marks NISX in this issue of the ORER.

Jim Brewer
Glenwood MD