"Which ex-MDC kit is this based on? #1340?
I'd think so. It's the #1330 with the ovals lopped off IIRC.
"Didn't the MDC #1340 gondola have side ribs which extended below the rest of the gondola sides? Or did MDC have another 40' gondola in their product catalog?"
According to the RPI site it's the only 10 panel 40' w or w/o the oval ends. The only other MDC/Roundhouse gons were the 50' Thrall Hi-Side Gondola, the 50' bathtub gondola and the 14 panel 'Railgon' 50' mill gondola.
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It's based on a C&O [oval] prototype.
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Eric