This 1984 G-III is fitted with QTA (Quiet Technology Aerospace) Stage 3 hush kits with their nice sci-fi look. The opening just aft of the reverser doors (shiny metal) is needed to allow them to be operated while still being able to thoroughly mix high-speed exhaust gases with lower speed ambient air in the aft duct during the rest of the operation. The latter is what actually reduces noise levels.