As a followup to my last post, I am pretty sure I found the original singer or even the original tape. I think at this point there are 3 possibilities.
There's a possibility that the original uploaded of the song pitched the song up and then sped it up. I tried doing that and it sounded extremely similar, including the instrumentals as well. If that's the case, then the original singer is 大野てる穂 Teruno Ohno. On the Doraemon no Uta Japanese Wikipedia article says that she recorded with King Records and Nippon Crown, however, so maybe the uploader put a fake cover as the thumbnail. This would also make sense, since the comments on the video say that they had the exact same album cover, just different songs (the official songs.) If anyone wants to try it out and give their opinion, transpose this video it by 3 and then speed it up to around 110% and it will sound very similar to the one allegedly from '74. Here's another one probably by Teruno, which also uses the instrumental and also sounds extremely similar. Speed this one up by roughly 115% and transpose it by 3.
There's a chance Teruno isn't the singer and whoever is singing here just sounds extremely similar to her and also used her instrumental track too. I will also note that her instrumental track is special in that the second chord, instead of a EMaj chord as in the original track by Kumiko Osugi, Teruno's is actually a E Minor chord, which I haven't seen in any of the other pachisons on Youtube, so her instrumental track is unique.
It actually is from 1974, which, as I covered in my last post, is extremely unlikely.
Some additional observations:
Doraemon's voice is extremely high in the purported 1974 tape, a lot higher than either Mazako Nozawa or Nobuya Oyama. So it really does sound like it was pitched up. If you pitch it down 3, it would sound a lot closer to the Oyama version, although it still is very high.
These are the ones to speed up by and transpose:
And compare to this one:
You can use this chrome extension to transpose and speed up quickly:
If you want to try for yourself feel free.