Seriously, very little 15 Big Ones contributions was new. "Had To Phone Ya" was already a Spring song. "Back Home" was 13 years old. "Susie Cincinnati" had already been the B-side to about 43 singles in 1971 alone. Heck, I even suspect that "It's OK" might have been recycled, as it's formulaic enough to have been an early '60s fun-in-the-sun tune. That leaves "Everyone's In Love With You," "TM Song," and "That Same Song" as the only ones I don't suspect as being old.
And the thing about "TM Song" and "That Same Song" is that they're simple and quirky enough that they might have been one of those songs Landy had Brian wrote about just random little everyday things -- more of which would make up a good deal of The Beach Boys Love You.
The band could have picked so much better older material instead of the music that ended up on the album. I wish "can't wait too long" was finished and the album was more of a "15 big ones" of great unreleased material from the vaults.
Songs that could have been finished and put on 15BO included 'Good Timin', 'River Song' and 'Angel Come Home'!