Up for sale is a remarkably well-preserved piece of 1970s audio history: a complete Koss K/2+2 Quadrafone 4-Channel Headphone system. This is a true time-capsule find, perfect for vintage audiophiles, retro tech collectors, or anyone building out a classic quadraphonic stereo setup. What’s Included: Koss K/2+2 Quadrafone Headphones: Features the original dual 1/4-inch audio plugs and the integrated 2-channel/4-channel selector switch. Original Koss Hard-Shell Carrying Case: Includes the original internal egg-crate foam protection in great shape. Original Printed Manuals & Literature: Includes the original “4 Channel Quadrafone” manual and Koss stereo booklet.