An interesting story from 1983 on stereo AM. Below are a couple excerpts as an overview, read the full story at: https://www.nytimes.com/1983/08/25/business/us-stops-sale-of-am-stereo-item.html
New York Times August 25, 1983, Section D, Page 1
FCC STOPS SALE OF AM STEREO UNIT
The Federal Communications Commission yesterday ordered the Harris Corporation to halt production of a device that permits monaural AM radio stations to broadcast in stereo and to notify its customers to shut their units down... ..differs markedly from the device approved by the agency a year ago.
In fact, said John A. Reed, acting chief of the F.C.C.'s Technical Standards Branch, the differences are so great that ''we're taking the attitude that this particular unit hasn't been type-accepted at all.''....
Ordinarily, the F.C.C. would be expected to select a standard from among competing and incompatible possibilities. But when it approved AM stereo transmission in March 1982, it specifically declined to make that choice. Mr. Reed summarized the commission's policy in this case as ''Let the marketplace decide.''
Harris is one of five companies - the others being Magnavox, Motorola, Kahn Communications and Belar - vying to set the standard for AM stereo transmission.....