Regional variation? When I was growing up (in California, in the 1970s and 80s), we called them chalkboards because, you know, you used chalk to write on them. Also, they were often green.
Or "tonic" (which was already a somewhat archaic term for soda generally in the Boston area when I was going to school). Boston had a number of those. Frappe was another--which you still see now and then at old-timey ice cream places. (A "milk shake" was just flavored milk.)
Do you call it 'soda' or 'pop'?