While looking for Dareka Watashi To lyrics, I stumbled on this See-Saw fansite – namely, a page that listed songs they performed live but never officially released.
Quick translation of the song descriptions (I skipped Chao Tokyo)
Mori ni itadakarete:
“after their debut they always did this song, and i thought it’d be in a single or album, but it became a phantom song. it’s uptempo but the lyrics are a little dazzling.”
Cars:
“here’s a song that they conversely did before their debut. after their debut they did it in their first tour and that’s the last time I remember it. it’s also uptempo, kinda like a coupling (B-side) song but sadly it hasn’t come to the light of day yet. [other stuff] and it makes you dance with the sound of a car engine starting as the intro.”
Kai:
“this is my favorite ballad song. magnificent feelings, there’s a part where the vocals really soar. Chiaki does a great job balancing it as it fades.”
Mune no Yukue:
“was deeply moved from the very beginning and I remember the melody being splendidly strong. they haven’t put male vocals as backing till now [can’t get this…maybe they put male chorus and it worked or something?] got chills in the English chorus part.”
Rokuda no Yume:
“hmm, kinda don’t have an impression left over from this one. it’s got feelings kinda like Mata Aeru Kara.”
Mania:
“from the beginning, the last careless piece. [something something] got an image like Dareka Watashi to.”
Himawari:
“hmm, this one was good. [etc.]”
So this means that (Kalafina’s) Mune no Yukue was originally written for See-Saw, and was performed by them live – in 1999, specifically. I don’t think there’s any doubt that it’s the same song, especially with “English chorus” being mentioned.
—by forum member ninetales.