Having noticed a hydraulic leak during the inspection conducted by the FIA, Mercedes had to replace the gearbox on the car of Lewis Hamilton, who then received a grid penalty.
The change, which took place on Friday night, provided an opportunity to examine the interior of the W09 transmission. On the middle image, we see the duct through which the muffler passes (white arrow, note the white insulating material), the transmission shaft (yellow arrow), a series of hydraulic cables, the oil radiator of the gearbox (blue arrow), etc.
On the Mercedes, as on the Ferrari, the gearbox is housed in a carbon casing which also contains various other components (oil pump, dampers, etc.).
The casing is the rear backbone of the chassis and features mounting points for the engine, wishbones, rear wing central pillar, and rear crash structure. Inside, the gearbox slots in like a cartridge.
First devised by Aldo Costa on the 2004-spec Ferrari F2004, the system has been copycatted elsewhere since then. Why is that? Well, while teams can only change the gearbox every seven Grands Prix, they are allowed to tweak the casing as they see fit (in order to modify the suspension geometry for instance). Therefore, the casing must not be replaced when a new transmission is mounted, as at the weekend.