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Robert SReal-Photo postcards refer to cards that actually are photographs, developed on photographic paper the same size and weight as traditional, printed postcards. Like other postcards, Real-Photo cards feature a postcard back complete with stamp box.
First popularized in 1901, these postcards can be identified by the images themselves, as well as by the stamp boxes, which identify the manufacturers of the photographic paper. More common names are AZO, Cyko, Velox, and later EKC and Kodak.
Because these cards could be made simply by purchasing paper and developing a photo directly onto it, most Real-Photo cards depict semi-pro and town teams. However, some of the rarest postcards of this type feature Major Leaguers, often homemade, in candid poses. Some publishers issued Real Photo postcards, as well, featuring major leaguers.
Unlike a printed postcard, the image on a "Real-Photo" card is not made up of print dots, it is solid like other actual photos.
The key to determining a "Real-Photo" card's age is the photographic paper manufacturer. Different brands, such as AZO, CYKO, ARTURA, VELOX and others all feature identifying marks in a card's stampbox, signifying when the paper was issued.
A complete "cheat sheet" and manufacturer stamp box photos are available at this link: http://www.playle.com/postcards.html#realphoto.
Here's a page of Real Photo baseball postcards:
http://www.vintageball.com/RealPhotos.html