Hey zam143 -
I'd feel confident saying it's from the 70's.
Even though WPL numbers were no longer issued after 1959. They were still in use in subsequent years.
The manufacturer of the t-shirt was H.L. Miller & Son out of Schuylkill Haven, PA.
The "All Stars" graphic I'm sure was something rather generic a high school or college team had printed up.
Hope that helps.
http://worn75.blogspot.com/