Postcard question - Shibe Park

[steve B]

I can't find the exact details, but I'm fairly sure it's because of its size and the postcard requirements at the time along with rates for different types of mail.

Postcards were 1 cent, but there were and are usually things that make them not count as postcards to the post office.
For example, before a certain date anything handwritten on the same side as the address made it considered a letter.

I suspect that being extra long, it would have not qualified as a post card, and would have required 2 cents postage. But marked as "printed matter" similar to a sales flyer, it would have still only required 1 cent.
Pretty slick work around, and if it was a generic topic I'd expect it to be worth more to stamp collectors than postcard collectors. That it's baseball takes that right out of the equation.