How is true "1 of#" determined?

the only "real" 1 of 1 or 'one of none' that I'm pretty sure of is the 65 Valiant Formula "S" that a member here or over on slantsix.org has. It was a documented special order "S" package on a Valiant for a Chrysler VIP. My car is one of one with its SO number...:-)