I may be wrong, but I remember getting self-serve Slurpees back in the 70s. Plastic cups with baseball players printed on them as I recall. You had a choice between Mystery Fruit Red and Frozen Coke. Mystery fruit Blue came later, followed still later by the 4 spout machines we enjoy now, you know.. the one that has three flavors working, and the one that you wanted a runny mess.
The signature element of pouring your own was the inevitable over-topping that led to a legion of flies following you home while you tried to wipe it off with your shirt sleeve.