There are articles about these devices in past issues of the "Chronicle", the journal of the USPCS. I have also seen them in auction catalogues (the device itself) with estimated sales prices between $200-$400. Covers and stamps with these devices used appear to be pricey. There are multiple designs. I have one with the American Eagle, with arrows in the talons above a field of 13 stars and two olive branches (minus the town slug).
These images are out on some magazine articles/advertisements.