A filter outer casing has a first set of interlocking formations around its outer circumference and/or its lower edge. Each formation is interrupted by a recess with axial extent. Alternatively the recess is adjacent to the side of the formation. The filter element (19) has a fluid-tight casing (16). It includes a second set of interlocking formations (36) on its exterior and/or its lower edge. When the filter element is inserted into the outer casing, the second interlocking formations are introduced into the recesses (43) associated with the first set of interlocking formations (42).
