Phosphates were carbonated beverages made-from-scratch by the "soda jerk" using carbonated soda water, flavored syrups and phosphoric acid -- in this case, they are flavored with strawberry syrup.

In slang, Strawberry Phosphates refer to strippers; the term is in reference to Thornton Wilder's play "Our Town", in which Mrs. Gibbs references her son George spending all of his money on strawberry phosphates, jokingly thought to be the character Emily's stripper name.
George: Aw, Ma,-- I got a lot of things to buy.
Mrs. Gibbs: Strawberry phosphates--that's what you spend it on.

"Our Town", Act I
by STAGEWEST December 20, 2011
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