First no cap, then a rounded cap that followed the rounded tip of the forend which was difficult to manufacture so was short lived, then no cap & then the type we are familiar with, most early No5s are with out the cap.
Pake, i would have expected your No5 to have the cap, yours may have an earlier forend on it thats be numbered to match & as they did not originally number the early forends we'll never know for sure.