1968 Looks like center lap belt was optional.
https://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1968/68_Dart0018.jpg

1969 Not as clear cut as the 68 book. Looks like 3 for the rear but what do the C13 and C14 mean?
https://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1969/69_Dart0021.jpg