In reply to Ed Gaida's posting about media mail [160404 MMDigest],
the handbook ("rule-book") for postal employees is the "Domestic Mail
Manual".  I like to include the chapter and paragraph reference number
on the side of the box I'm mailing in case anyone complains.  Item
4.1e specifically states "player piano rolls" so on the side of the
box I typically write

  Media Mail
  DMM 173/4.1e

The entire manual can be viewed here: http://pe.usps.com/text/dmm300/173.htm 

Adam Aceto
Burbank, California