The great conversion from mono to stereo was traumatic. In 1958 all AM stations were still mono as were FMs. But in just a few years, home stereo systems were switching over to stereo even without a clear technical standard. The reason was that radio was feeling a little peer pressure in the stereo cabinet.
LPs were going stereo. Western Electric had pioneered the Westrex process called 45/45, a single groove stereophonic system. The important part is that existing monophonic equipment could still play the Lps ensuring instant consumer acceptance. It worked. Western Electric was the manufacturing arm of AT&T for the better part of a century. They started out in the 1850s making typewriters.
In the 1970s, the now flat-sounding mono records were commonly rereleased with "enhanced" sound. this was also called "Duophonic" sound after the Capitol records process. These were marketed as stereo versions of the original releases. But the monophonic master tapes had no stereo separation. So they had to fake it. The stereo effect was created through a couple remarkably simple techniques. Some audio filtering was used to separate out certain sounds to pan them. This usually produced noticeable audio artifacts. Also common were slight adjustments in EQ and phase which is what RCa seemed to focus on. An offset of 20 and 50 milliseconds was another very rudimentary change that mimicked the stereo effect.Worse yet was over-dubbing. A small trio would be brought into the studio to overdub minor parts to add stereo sound to the mono recording. There is an abominable Roy Orbison LP out there with a dubbed in accordion track that is a testament to the general badness of this idea. Also interesting is that as Stereo picked up, these old mono recordings went out of print. It left both radio and record collectors forced to either play the new clunky "enhanced monophonic" Lps or move on. Mono was essentially dead by 1978, but enhanced mono dragged out straight through the 1970s. Some Labels carried Mono, ST (Stereo) and DT (Duophonic) notations, but the collections were usually a mix of all three regardless of labling.