That pack must have been issued after the season was over, sort of like those infamous 'Christmas' packs. The graphic on that pack is the same one used during the season, only it didn't have that "660 cards" verbiage on it.
1973 Topps Wrapper.jpg
I have never even once heard a person claim they were all released at the same time. Look anywhere--the PSA website, Beckett, etc.--they all say the same thing: they were issued in series. That is why Beckett always listed them in separate series.