When recording in stereo, the machine tapes two tracks in your terminology HIFI L, HIFI R, in a helix pattern on you tape along side you picture, it also tapes a mono track along the bottom in a straight line. It will always tape this mono track (generally a combination of the other two), but not necessary record the stereo ones(as these can be switches off). On play back your machine will play the stereo track unless you switch them off, then it de-faults to the mono track, the mono track is generally of poor quality, but should not be heard unless the other tracks are switched off.
If you dub the sound track of a tape, you cannot over write the stereo track, only the mono track on the bottom.
I use to have a machine that was brought out just before NICAM broadcast and had simulcast, (it was also was NICAM) this was so if there was a stereo concert on TV and on the radio at the same time you could tell it to tape the picture off TV but tape the stereo off the radio. This was a great facility as I could go out tape a stereo radio program, and tape a program off TV at the same time. Come back listen to the radio program, switch the HIFI L, HIFI R tracks off, then watch the TV program, albeit rubbish sound quality. If it played all three tracks at the same time I would be getting the TV program whilst listening the radio recording.
Try playing the video and individually switching each track off and listen to the sound quality.
Hope this make sense and helps.
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