Chronologically, the events take place immediately before chapter 1.
It was originally published and released in the Mobile Suit Gundam: The Origin: Official Guidebook in 2004, it was later reprinted in the December 2006 issue of Gundam Ace. Chapter 59 was in the November 2006 issue and all subsequent releases also place it after chapter 59. Thus, it is the conclusion of the flashback arcs.