Just an engine removal to lighten the aircraft in preparation to be slung out from a landing site...or being cannibalized for parts.
Missing is the engine and engine Transmission and the lower sheet metal fairing to the Upper Pylon under which rests the Combining Transmission which you can barely see the Output end.
The Synchronization Shaft runs from the Combining Gearbox to the the Forward Transmission.
If something were to cause a failure between the Aft Transmission and the Forward Transmission.....the Chinook becomes the World's fastest Sausage Maker.
Preflight attention to that part of the helicopter involved at least two crew members or sometimes three....just to make sure everything was as it should be.