RP smoker's hatch

[Oxford Jack]

I may be absolutely incorrect so do not take this as gospel, I was under the impression that the smokers hatch was fitted to fixed head bodies as a ventilation aid. In the case of sliding head there would be no need of the additional hatch. The opening should face aft inorder to avoid becoming a rain scoop, but that is conjecture on my part rather than from actual experience.
Now heading for the hills to avoid the incoming!