In episode 194 of the anime, Princess Kakyuu speaks of a war and of a legendary soldier.
Kakyuu: Let
me tell you the story, of the Holy War from a long time ago that engulfed
the galaxy. Everything about the Sailor Wars!
Mars: Sailor... Wars?
Kakyuu: Sailor Soldiers are born of stars and planets. They've been fighting since the creation of the galaxy against those who infringe on peace. Until at last, the origin of evil throughout the galaxy, Chaos was defeated and sealed by the legendary strongest Sailor Soldier?
Kakyuu goes on to tell about the "Light of Hope" left by this soldier.
During her speech a scene is shown that depicts a shadowed soldier with a uniform
similar to 
those
of the Earth soldiers. Of course, at that time we don't know that Galaxia is
the legendary soldier, only near the end of the anime is this revelation made
known. The animators probably didn't want us to know that it was Galaxia, though
they did give a few hints here and there.
Later in the series, Galaxia also speaks of when she sealed Chaos and the Sailor Wars, but we don't see anything of that outfit from when Kakyuu told her story. Galaxia, rather, was in her golden armor-like outfit that she is always in. I still find it fun to speculate on the flashback outfit, but it should also be kept in mind that it probably was never one of the outfit's Galaxia had. Except maybe in fanfics.