Here is a response I composed for the Rockin' 50s message board run by Bill Griggs on the topic of overdubs added to Buddy Holly's demo and apartment recordings:
In the 1960s record labels would do anything to make low fidelity recordings sound like high fidelity. Stereo was new and being accepted. Multi-track technology was making lots of new developments throughout the decade. Consumer demand was there, too. Had record labels released demos and air checks without enhancements, there would have been complaints. It was not like it is now where consumers want vintage recordings to sound as much like originals as possible with as little evidence of deterioration as possible. These days people tend to know what they are getting and like it that way. Back then, most everyone wanted everything to sound brand new like it had been recorded that year. Thus record companies would overdub new instruments and voices, usually in stereo and overdub them onto older mono recordings, some being masters, others being demos or air checks.
Most often, though, mono recordings would be filtered into fake stereo, tuning out treble one one stereo channel, and bass on the other. This effect was way too common through the end of the 1970s. It almost always sounded terrible and to me physically painful when heard with headphones. Even worse, the RCA label would, in addition to this, DELAY the right stereo channel a bit from the left creating a crisscrossing effect. Way too many Elvis Presley, Sam Cooke, and God-knows how many country LPs had this effect applied. Thus, Elvis or Sam could be heard harmonizing with themselves, totally different from the mono 45s or older mono LP pressings.
Some Sun recordings made at 706 Union had stereo overdubs made at the new 639 Madison studio, Charlie Rich's "Lonely Weekends" being one.
Duck Dunn and Al Jackson re-recorded their bass and drum parts in stereo to make early Otis Redding mono recordings into stereo. Their efforts were subtle and don't sound too bad.
B. B. King's earliest mono records on the Modern label had big band overdubs in stereo added on LP releases in the late 1960s to try to make these records sound like his then current LPs on the ABC label. The orchestra used was of exceptional quality (even making me wonder if it was Count Basie's orchestra making extra bucks but PLEASE don't hold me to this. This is how rumors get started!). Listen to these same mono originals on current CDs. They sound so incredibly good. Why would anyone want to add to these? Well, it's 'cause they were not in stereo and they had to be in stereo so they could put the word "STEREO" in big letters on the cover so people would buy them.
Hank Williams SR had too many demo and masters overdubbed in stereo with ever increasing numbers of added instruments as the years went by. They just ruined Hank's original sound.
At some studios, mono recordings were recorded in two tracks, instruments on one channel, voices on another to facilitate better balancing at the mix down stage. Yet when the demand for stereo came around these two track recordings, never intended for stereo in the first place, got released in stereo. The all voice track on one side and all instrument track on the other sounds, again, just really awful. This happened to lots of early Beatles records on American LPs.
Fake live LPs happened then, as well. Existing or new studio recordings would have fake audience applause grafted on. Some labels just couldn't manage recording concerts.
So, it was the same when it came to putting out Buddy Holly's numerous demo and homemade recordings. These days, efforts would be made to restore these recordings to their original sound, but in the 1960s, like everyone else, the recordings had to be in modern sounding high fidelity stereo. I prefer to hear the undubbed versions when ever possible, but I actually like the Fireballs' overdubs. The Jack Hanson group can be heard struggling to follow Buddy's momentary breaks in rhythm. The Fireballs sound much more confident and seamless. I am particularly fond of "Crying, Waiting, Hoping". George Tomsco is a great guitarist and I think the whole group, under the direction of Norman Petty, sound fine on the overdubs. So much so that I think had Buddy lived he should have recorded an entire LP with Fireballs as the back-up group. I think it would have given Buddy a proto-surf sound which, in the early 1960s, would have modernized his sound a little and would increase his chance of new Top 40 hits on the charts and on radio.
At any rate, I really dislike the things done in the 1960s and 1970s to try to make old recordings sound new.