Double s 301 made their unofficial debut under DSP Media in 2008 with the EP, "UR Man," as "SS501 Special Project Group" in the absence of SS501 members, Kim Hyun-joong and Park Jung-min.

In 2015, the three members signed under CI Entertainment, who rebranded the group as Double S 301 before releasing their official debut album, Eternal 5, the following year.

[1] In 2008, both Park Jung Min and Kim Hyun Joong were given solo projects for the Grease musical[2] and the Boys Over Flowers series,[3] respectively.

[4] They also contributed to the Boys Over Flowers soundtrack with "내 머리가 나빠서" (Because I'm Stupid),[5] and appeared as a cameo in episode four, performing their track, "U R Man".

Even though plans are not finalized yet, the members are very determined and our side is also very supportive of it.” Heo also confirmed the comeback during his 'Hello Again' fan meeting.

Also, it's a nod to SS501's Official fanclub, Triple S.[10] The group was set to have a concert at BLUE SQUARE Samsung Card Hall on March 19 and 20, 2016 entitled "2016 Double S 301 CONCERT〈U R MAN IS BACK〉IN SEOUL".