I buy and sell thousands of postcards per year (not baseball)...there are commercially produced real photo postcards..mostly European. I have never once seen a printed postcard with an AZO back. There are photos of photos, photos of paintings, and just about any other thing you can think of printed on real photo paper.
Here is a recent example of a Titanic real photo postcard I sold that was just a photo of a painting...I got 5 of these in a recent estate auction so they were most likely made for commercial purposes, but almost surely were made by a photographer to make a few bucks on the disaster.
http://www.ebay.com/itm/370715700734...84.m1559.l2649