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...
Bunch of possible reasons: attrition, airframe fatigue, not enough IIIs to go to all squadrons. Real world, we had two squadrons of A-7s still flying in the Gulf even though the Hornet had entered service 8 years early. More recently, Navy/Marine F/A-18 squadrons have switched from C models to A+ because the Alpha+ had more airframe life than the charlie stockpile. Plus, newer pilots tend to get older gear.