Some things I find interesting.
 Hope you also do....

 
Disabled American Veterans (DAV) Covers 
 
This is a cover I received on October 16, 1998 franked with four copies of the 5¢ Circus Wagon (Krause-Minkus 1077, Scott 2452D) and four copies of the 3¢ Conestoga Wagon (Krause-Minkus 883zxs, Scott 2252a) 

Notice that this return envelope to DAV contains the bar coding and the FIM necessary to qualify the outgoing solicitation for automated mail discounts. 
 A nice example of DAV franking....  but this one is with a "bonus"

The Circus Wagon strip of four has an S2 Plate Number Coil (PNC) in position one.

 
For years DAV has used low denomination coils on the mailings.  Since USPS now produces only untagged versions of these stamps, the contributions sent to DAV in these return envelopes do not have the tagged stamps necessary to activate the USPS facer/cancelling machinery necessitating non-automated handling.  Reports in the philatelic press indicate USPS has been requesting DAV to add some sort of taggant to these return envelopes and that DAV has refused due to the added cost.

Perhaps by design but most likely by pure chance, the Circus Wagon stamps on this cover are printed with the red ink which slightly responds to both long and short wave ultraviolet light.  Collector initiated tests several years ago showed that these stamps did receive more machine cancels than did completely untagged stamps but less machine cancels than the tagged versions.