I just received a book from a PBS member. Looks normal expect there is a plain yellow/green sticker next to the stams with a standard style circular date stamp like they use at the counter. Ihas a USPS logo, the date in the center, the post office name around the bottom and around the top is says: AVSEC UNIT.

Figured out that AVSEC Stands for "Aviation Security" - a program established in 2001 or 02. They are supposed to check mail going on planes for hazardous material (bombs???). Strange thing is this is marked media mail.  Could it be...be still my heart...that this media mail travelled by air to me. Probably not, since it took 7 days.

Or...could it be the only postal inspectors in the area are in that unit and they also spot check media mail???

Anyone else seen such a marking?  The package did not appear to have been opened, just that additional marking (the stamps were all cancelled with a different looking CDS.

wc

Perhaps your local post office missorted it, and put the package in the mail bag for first class items, so it was processed there first before being sent along it's proper path... happens sometimes.

If it weighed over 13 ounces and was mailed from an office that uses stamps for postage, not meter strips or PVI labels, then it has to be stamped with the AVSEC stamp to show that it was accepted in a face to face transaction, not just left on a dock or dropped in a collection box.  

Also the office is not supposed to use the AVSEC stamp to cancel the postage stamps.  They are separate and distinct markings.

If it weighed over 13 ounces and was mailed from an office that uses stamps for postage, not meter strips or PVI labels, then it has to be stamped with the AVSEC stamp to show that it was accepted in a face to face transaction, not just left on a dock or dropped in a collection box.  

The color and shape sound similar to stickers used to validate  the 13 oz. rule, so I'm liking Diane's answer. 

Never seen it, but when I worked for Fed Govt and lived in MD near DC, our mail coming from certain locations was irradiated--yep we lived thru the anthrax scare!   All the envelopes felt "cooked" and brittle and they were stamped with a special large red marking.