These are a good catch, and I congradulate you, but the only NCO artifact is the cofin tank (not including the flat car it rides on.)
The flat car under the tank is a typical Westside Lumber Co flat, as one (in pieces) of the other three flats. The two SPNG cars were just that, SPNG cars... one built new circa 1917 for the SPNG using new parts, and the second many times rebuilt from a flat built originaly for the Origonian RR by the Carter Brothers in 1880, later to the SPC, then to the N&C which became the SPNG. Along the way the Oregonian car lost its trucks, got air brakes, lost its link & pin couplers in favor of Sam's then knuckles, lost all of its original stake pockets and all the wood. It still retains possibly original center plates and draft gear stop blocks.
We have the patterns to make many of the replacment parts for these cars in our collection, and would be glad to help
Randy Hees
Socety for the Preservation of Carter RR Resouces