Image: from an auction posting on ebay.com
Amateur radio has a long and storied history. By 1939 it was a well-accepted hobby to the point that operators often had postcards printed with their call letters and related radio information. When they would reach a far-away listener, each would send a postcard to the other with the details. In this example, Howard D. Childers of Los Angeles, California was able to communicate with David L. McClelland in Masontown, Pennsylvania. It wasn’t unusual for the hams to display the cards they received on the walls around their radio set like wallpaper.
Learn about the history of amateur radio here: http://w2pa.net/HRH/
“RCVR” at the lower right of the front of the postcard means radio receiver. “Sky Buddy” was a brand of receiver made by Hallicrafters. Learn more about it here:
“XMTR” means transmitter. Chances are it was assembled by the operator from a variety of components. It might have looked something like this: https://www.qsl.net/ab0cw/2a5.htm .
The term “ham” was originally a negative term describing one or more operators doing things that prevented others from using the airways. For a more-detailed explanation, go to http://www.arrl.org/what-is-ham-radio/# then click on “Why are Amateur Radio Operators called ‘Hams’?”.