My postmistress refuses to consider metal music box discs as qualifying
'media mail'.  Her reason: "They are metal."

I've read the MMD postings re Media Mail & player piano rolls and I'm
wondering if anyone has knowledge of authoritative rulings on music box
discs, or any other references/quotes that would help me persuade her.

Jack Perron - HensTooth Discs
http://www.henstoothdiscs.netfirms.com/