The SLR port is the cylindrical shape front of the housing that contains the camera lens. The port removes easily and is available in various lengths to accommodate different lenses. A dome front is utilized for wide angle lenses, and a flat front for macro and longer focal lengths.
The seal of the interchangeable port system is the only design that allows you to see if it is correct. You can look through the clear plastic and see the "O" ring seal. If it looks sealed it will be sealed. Every other SLR housing relies on theory and hope. There is no way to see if the port is sealed properly on an aluminum housing.
FLAT PORTS are for standard, macro and telephoto lenses.
The 5505.46 works Auto focus only.
Flat port recommended for macro lenses