Update on the car's history: Apparently, the BN got them used from the Valley and Siletz (VS). The reweigh date, and probably when the reporting marks were changed, looked like ?-84.
When the VS received them I don't know, as I couldn't find a picture of these cars in their paint. Similar double sliding door cars, which apparently also went to BN, seemed to be dated 12-79, but the pictures weren't clear.