Wasn't there a bootleg set of US singles quite recently, which amounted to 6CDs? Wonder if the "16" was a mistyping? I can't see Capitol EMI putting out a 6 CD set.
Found it... the sixth disc was a bunch of promo-only tracks and other slight oddities. (Excuse the formatting -  this is a crude tidy-up of a cut & paste).  The full line-up is on Bootlegzone.com and yes it does appear to be a fan compilation rathe than a silver disc boot.
Disc 6
1    Add Some Music To Your Day   (B. Wilson / M. Love / J. Knott) 3:37
 Promotional Single Version  
 The Beach Boys
(date unknown)
2.   Susie Cincinnatti   (Jardine) 3:02
Promotional Single Version
(date unknown)
The Beach Boys
3.   Tears In The Morning     (Johnston) 3:12
Promotional Single Version
(date unknown)
The Beach Boys
4.   It's About Time   (Wilson/Burchman/Jardine/Wilson) 2:55
Promotional Single Version
(date unknown)
The Beach Boys
5.   You Need A Mess Of Help To Stand Alone   (Wilson/Rieley) 3:27
Promotional Single Version
(date unknown)
The Beach Boys
6.   Cuddle Up   (D. Wilson / D. Dragon) 5:29
Promotional Single Version
(date unknown)
The Beach Boys
7.   Almost Summer   (B. Wilson / M. Love / A. Jardine) 2:31
Promotional Single Version
(date unknown)
Celebration
8.   Here Comes The Night   (B. Wilson / M. Love) 3:22
Promotional Single Version
(date unknown)
The Beach Boys
9.   Hold Me   (C. Wilson / M. Smith) 3:31
Promotional Single Version
(date unknown)
Carl Wilson
10.   Night Time   (B. Wilson) 3:07
Promotional Single Version
(date unknown)
Brian Wilson
11.   Somewhere Near Japan   (Phillips/Melcher/Love/Johnston) 4:13
Promotional Single Version
(date unknown)
The Beach Boys
12.   Problem Child   (T. Melcher) 3:44
Demotional Single Version
(date unknown)
The Beach Boys
13.   Forever   (Wilson/Jakobson) 3:00
Demotional Single Version
1992
The Beach Boys (Wee Helper's guessing the Stamos version?)
14.   Forever   (Wilson/Jakobson) 3:06
Demotional Single Version
1992
The Beach Boys (likewise -  maybe a Euro-remix?)
15.   
Under The Boardwalk   (A. Resnick / K. Young / M. Love) 3:29
Demotional Single Version
(date unknown)
The Beach Boys
16.   
Summer In Paradise   (Love/Melcher/Fall) 4:10
Demotional Single Version
(date unknown)
The Beach Boys
17.   
Summer In Paradise   (Love/Melcher/Fall) 4:29
Promotional Single Version
Concert (date unknown -  Wee Helper's guessin' the way-superior-to-the-album version recorded at, I think, Wembley, London)
The Beach Boys
18.   Don't Worry Baby   (Wilson/Christian) 3:22
Promotional Single Version
(date unknown)
The Beach Boys With Lorrie Morgan
19.   Sloop John B   (Traditional - Arranged By B. Wilson) 3:42
Promotional Single Version
(date unknown)
The Beach Boys With Collin Raye
20.   
The Warmth Of The Sun   (B.Wilson/M.Love) 3:15
Promotional Single Version
(date unknown)
The Beach Boys With Willie Nelson
21.   Caroline, No   (Wilson/Asher) 3:15
Promotional Single Version
(date unknown)
The Beach Boys With Timothy B. Smith
22.   Little Deuce Coupe   (Wilson/Christian) 2:51
Promotional Single Version
(date unknown)
The Beach Boys With James House
23.   409   (Wilson/Love/Usher) 2:20
Promotional Single Version
(date unknown)
The Beach Boys With Junior Brown