Hiya Paul, the postcards were just a freebie at the Nevada Club. I've only noticed slot players in the pics. They'd take a photo and make it into a postcard and present it to the subject as a momento. Don't think you necessarily had to win a jackpot to get one. They'd also monogram a book of matches for you as everyone smoked back then. The matchbook thing was also available at the neighboring joints and the different incarnations of the Nevada Club on Fremont.

Here's a couple from a series of six I snagged on eBay that portrays several sisters palying the machines in '58 and '59. Great fun to make up little stories to go along with images. A pretty effective way to gain insight to the context of the gaming chips of the day. BTW, notice the sign across the street!