Five Cents Plain

I have heard and seen the expression "Five Cents Plain" scralled in chalk on overpasses when I was a kid butI believe it belonged to my father's generation. I have always believed that it had to do with bulk candy that used to be sold in apothacary jars in the old time candy stores. Does someone hear know the actual meaning?

"For Two Cents Plain" is the title of a book by Harry Golden. It refers to the price of a large glass of seltzer water without flavoring, during his childhood. Apparently the price has gone up.