my problem with the collapses into forearm theory is: where does the hand go?
I think that the weapons may fold down to be more flush/protected when not in use but is still not standard to the yf-19.... I figure if it was, isamu would have used during his dogfight with guld.
the hand doesnt realy need to retract into the arm, but yah looks to be a potentional weapon system test like a cop's hide-a-way pistol.