Battle, you mentioned Fleer World Series in your memory around the time you may have come across some 16ths. I believe those came out in 1979, so maybe there were some candy stores or ice cream trucks that had 16ths floating around for a couple years. It stands to reason that sales were slow, as most of us kiddies probably figured the party was over after the 15th, given whatever hiatus period OS Wackys had taken before their last-breath comeback.
I experienced something similar around 1977-78. A new candy store had opened in the neighborhood, and the guy had boxes and boxes of Monster Initials, but those really dated back to 1974. How did he get so much of it, especially in a newly opened store? Just another little timeline mystery there is no answer for.