I'd buy that theory.Wikipedia says this:
The transformer division ended up as part of the Cooper Power Systems division.
In 1967 Wagner Electric was merged with Studebaker and Worthington Corporation to create Studebaker-Worthington, a diversified American manufacturer.
Not sure if that means they would have stopped putting "Wagner" on the transformers in 1967, but seems reasonable.
If it had a 2-digit postal zone then that would imply it was before 1964. But because there is no postal zone or ZIP code provided I think it remains ambiguous.No zip code, so that implies before 1964.