What a great point. In my research, I saw no DFP cards that appeared to be postally used. Not all online images showed the backs, but none of them had any signs of being postally used like a postmark. What does this mean then? I can see it's significant, but can't put my finger on why. And if they weren't distributed to the public, what was their purpose? I found this old Lew Lipsett auction where Jeff's Cobb sold many years ago. There were 11 total DFP cards auctioned off in four different lots. Lew did not provide pics of the backs. Here is a link. There was also a Rose Co Wagner sold for just over 5k...those were the days.
http://www.oldjudge.com/archive/200811/postcards/#205