The lens turret holds 3 lenses. The one on the left is the one in the "active" position.
Lenses are removed by squeezing two levers, above and below the lens. There are also levers for rotating the turret mounted between the lenses, and the focusing rings on the lenses also have short "wings" on them.
The lens fits quite a ways into the body of the camera, the two levers fit into the groove just before the lens starts tapering on the left. There is also an opening in the lens for a tooth within the camera, which keeps the lens from rotating.
The matte box then fits in front of the lens. There are two filter slots on the matte box, directly infront of the lens.