While going through my Afghanistan stamps, I managed to find a group of stamps that all had Washington, DC cancels. I initially found this strange and took to some research to figure out what these were.
It turns out that these stamps were affixed to letters sent to the United States in what is called “Diplomatic Pouch Mail”. Probably from the US Embassy in Afghanistan.
The letters were grouped together in a pouch and then sent all together to the US, where, upon receipt in Washington DC, they were postmarked.
The below image shows an example of a few full covers (taken from an eBay listing on 11/20/2014)
The return address lists “The Foreign Service Of The United States of America” which is the US State Department.
These covers were priced a bit out of my reach ($75), but hope to find a few in the future at a better deal.