Gravel Kit
This is a gravel deflector kit which will allow the aircraft to operate from unimproved airstrips. The plate, made from fiberglass coated plywood, keeps gravel from being sprayed back into the engine intakes. There is a different gear extend/ extended speed (which I've long forgotten).
There is another kit that is made up of two stainless steel booms that extend from the underside of each engine. These are vortex dissipators. Bleed air is tapped off and the air is literally blown onto the ground just under each engine intake. This has the affect of negating any vortices created by the engines which then protects the ingestion of any rocks, sand or debris into the inlets.
With the gravel kit in operation, the resulting EPR penalty was only something like 0.01 EPR.