Quadraphonic

Quadraphonic sound – equivalent to what is now called 4.0 surround sound – uses four channels in which speakers are positioned at the four corners of the listening space, reproducing signals that are independent of one another. Quadraphonic audio was the earliest consumer product in surround sound and thousands of quadraphonic recordings were made during the 1970s.

Usually made only in small numbers so highly collectable and usually better sounding hence the lower numbers of pressings.