Pocket Music = ポケット・ミュージック

Pocket Music, Tatsuro Yamashita's first digitally-recorded album, has been reissued multiple times with a variety of remixes.
Tracks 1 ("土曜日の恋人"), 3 ("Mermaid"), and 10 ("風の回廊") were originally released as 7" singles in 1985. 風の回廊 (コリドー), released in March, was unusually mixed by Yamashita himself. 土曜日の恋人 / Mermaid were released in November 1985 with mixes by Yasuo Sato, Yamashita's main engineer at the time.
The original 1986 album mix, released on 23 April (LP/CT) and 10 May (CD), was mixed by Toshiro Ito, retaining only Yamashita's "風の回廊" mix. This mix was disliked by critics of the time for its 'cold, digital' sound that sacrificed some of Yamashita's sound profile in favor of mid-80s industry trends (transient drums, unnaturally bright digital effects, and focus on digital dynamic range). It was discontinued in 1991, but its "土曜日の恋人" alone was reissued as a bonus track on the 2020 remaster.
In 1991, Yamashita's original engineer Tamotsu Yoshida was hired to remix all tracks except "土曜日の恋人" and "風の回廊." Yoshida's mixes align closer to his previous work with Yamashita - most likely the reason the album was remixed. For this reissue, Yamashita opted to restore the original "土曜日の恋人" mix and again keep his own "風の回廊" mix; however, "Mermaid" was remixed for unclear reasons. For tracks 2-9 (and "My Baby Queen," an outtake added to incentivize fans to buy the album again), the 1991 remix is the "final" mix.
In 1995, "土曜日の恋人," among other songs bound for greatest hits compilation Treasures, was again remixed by Yasuo Sato. This remix is essentially a recall of his original mix, with a louder electric piano and alternate ad-lib just before the final verse. The remix was not included on the album's 1999 CD reissue, as that reissue was simply done to update the catalog number.
Beginning with Opus All Time Best 1975-2012, the 1995 'Treasures Mix' of "土曜日の恋人" has functioned as the final mix of the song. The 2020 remastered CD includes both of the older mixes of "土曜日の恋人" and a karaoke version of its 1995 mix as bonus tracks, as well as Sato's original 1985 "Mermaid" mix. In total, "土曜日の恋人" and "Mermaid" have received three mixes; "風の回廊" and bonus track "My Baby Queen" have received one; and all other songs have received two. As of 2020, all mixes are available on both vinyl and CD.