Bucket seat rail fasteners needed

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

Dave,
That would be me.

I just sent a set to a customer for use on a 71 duster and his fit. This is the 1st I've heard of a larger size stud for the later cars.

feel free to send them back no question

Are you sure they are the oem studs? You wouldn't be the first guy to find carriage bolts in that spot.

My 73 Dart Sport has 5/16 studs on the bucket seats.

Sounds like the right nuts & the wrong studs to me.

No, I've owned the car since '76, so I'm pretty sure the bolts are original. I know I've never changed them out. This is really weird. Do the bolts only come in one thread configuration? Is there a different bolt for a bench seat car than a bucket seat one (I have buckets)?