Factual error: When they go to 1954, the hamburger drive in says it has frappes. Not invented till '57, and not in the US public eye until at least 45 years later.
Suggested correction: What you're describing is a Greek café frappé made with instant coffee. In New England "Frappe" is what many would consider a milkshake with ice cream in it, and has been around since at least the 50's (Vic's dad even mentions getting frappes in episode 4).