It's a mystery. I never knew that they had planned an Archetype model. I own Big Duo (unfortunately, not the Roger & Angel w/ wing extensions that came with the set) and the Big O model (one freestanding and the other still in-box thanks to PCF
).
Big O was originally made to be a toy, and the show was supposedly produced to help market an action figure. With such a plethora of mechs (the "monster of the week" giving us a healthy number of robots) I don't know why only two ever went into production... and then only Big O was produced in any sort of healthy amount (Big Duo was a limited run, and Big O had the additional assembly-required model). The "coin" figure sets had some amazing detail, albeit imposable figures and even included Dorothy-1.
Supposedly, Big Duo and Fau were supposed to be made along with a new O back in... what? 2005? Never got out of the prototype phase. The only success since then has been the Soul of Chogokin model (fully articulated die-cast model) along with a limited run expansion set (O-Thunder and Hip Anchor add on with alternate "faceless" head). Both are exceedingly expensive, and completely worth it (I still need to buy the expansion). I can only hope that Soul of Chogokin (also made by Bandai?!) decides to make Big Duo or... better yet, Big Fau. Hell, I'd even take Beck Victory Deluxe!!