Well, now, there's still more collateral material to be had for this series. Here are the publicity photos sent by the USPOD to newspapers and philatelic publications, as well as to the FDC makers of the period, giving everyone a chance to see what was coming from the USPOD. Here are a couple of examples.
Each of these would have been accompanied by a short, rather terse letter, such as the one shown here:
Another area you can try to find are the post cards announcing the issuance of the stamps. These were catalogued by Dr. Cary Binder. The APRL has copies of this in their stacks. Here's an example of the post card:
Then, if you really, really want to look for a needle in a haystack, try finding a top deck card for each of the issues. Here's the only one I've found so far, the one for France: