They were a factory option though (or maybe just Dealer Installed as mentioned) and have their own unique Buckets and Emblems which I believe are 1 year only. The '64's seem to be very similar, but have a different Emblem on them. In my experiences anyway. Many times different plants had different parts. I'm sure there are Seat Belt experts out there. If no one jumps in here, try Chevytalk.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America