The Velocity deck has a much larger discharge outlet and some minor differences in the removable/adjustable baffle across the front. Scag machines with Velocity decks have the adjustment on the left front support chain.
I sometimes mulch with my 48" Velocity deck using G3 blades and a blocking plate. I also use the G3 blades all the time, mulching, side discharge or collection system installed.
I tried G6 mulching blades, but the airflow created by them causes extreme blowout, G3s not near as much.