Introduced in Blood and Chrome. Very similar to the Mk II so I didn't spot it at first. The fuselage seems more bulky, the cockpit canopy seems more rounded and the wings slightly larger.
Didn't think we'd see anything else but I thought this was a nice easter egg considering it'll probably be the last BSG we'll see for a while.
Does raise the question why Mk IIs are still being used in Razor...