This is a still unfinished housing, as it is when it comes from casting. The round shape is typical for the fully automatic gearbox, because this consists practically, of only one (quite often divided) shaft. Therefore, it is considered, by the way, as being simpler to dismantle than a manual gearbox, whose case encompasses at least two shafts and a small return-wheel, in the profile it resembles more a figure '8'. To the rear of the gearbox exit, the diameter becomes smaller. In the front at the bell-housing, around the torque-converter it is much larger. Under this casing is also the quite extensive electronic regulation systems unit. It is sometimes even broader than the gearbox itself.
A look at the insides, reveals the clearly visible key-slots. This is where a multi-disc clutch takes hold, which brakes part of a set of planet gears when accordingly activated. Therefore, it should probably rather be called a disc-brake. 10/08