a) at the top, the three shifter rails that are operated by the gear shift lever. The one on the right is sticking out as the gear selector is in first.
b) in the centre, the main output shaft from the transmission that will connect to the pinion gear via the short shaft which you can see in the picture of the differential housing.
c) sticking out toward the rear, the shaft that engages with the pto lever. The end of the lever sits between the two flanges on the shaft; moving the lever back causes the shaft to move forward, which moves a fork, engaging the pto.
d) at the bottom, the receptacle for the end of the pto shaft. The yellowish part is the cage of the ball bearing.
Just above the gasket at the bottom you can see some corrosion - the UK tractors' gearboxes are made of some kind of aluminum alloy. Because of this (I guess), the gearbox-to-differential case bolts are coarse-threaded where they thread into the casting, but fine-threaded where they have nuts.
