A formed letterbox is an aperture that has been physically constructed within a wall construction using stud and boards of the same specification as the wall itself. Otherwise known as a framed and lined letterbox, a stud is used to create a frame, and then the inside of the aperture is lined with plasterboard. The fire stop seal is then installed inside the opening.
If the aperture in the wall has been roughly cut out and not formed (framed and lined), then it may be necessary to fit the fire batt fire seal to the outside of the wall. This is what’s known as a pattress-fit or face-fix batt system.
In any case, apertures should be created in accordance with the wall manufacturer’s recommendations based on their tested scope of application.