I agree with you that the east side was heavily European ethnic and heavily Catholic [[as many teams in the Catholic high school football league as in the public high school football league. European immigration pretty much stopped in the early twenties except for the displaced persons diaspora from eastern Europe post WWII.
The immigrants began to flow out of the city after WWII though and were replaced by blacks and hillbillies. For evidence, check out the Warren phomebook. It is chock full of Slovak, Ukranian, and Polish names.
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