Take a look at this, from an eBay listing:
These are Sony Metal Master audio cassette tapes. “Metal” refers to the type of coating/particles on the tape. If you’ve ever handled cassette tapes, you’ve almost certainly seen Type I, the basic tape for voice recording, or maybe recording off the radio. You’ve likely seen Type II tapes, the ones intended for music, and frequently labeled “For CD” or something like that, in the 1990s.