I just purchased some "Mopar OE" bucket seat rail fasteners from an Ebay vendor for my '73 Dart Sport. The washer/nuts (5/16x18 threads) were advertised to fit "factory bucket seat equipped Dodge and Plymouth A,E,C or B body vehicles." However, I couldn't get them to screw onto the bolts under the floor pans on my Dart -- apparently they're too small. Does anyone know if there were different threads on these fasteners according to different models/years? I'm trying to find the correct anodized washers/nuts to fit my vehicle.

Thanks