Rob,
I had three of them at one time and two of the three never had one installed. I have one here that does have it but it's a rifle that was only surplused from Australianicon Army stores in the early 1980's according to the previous owner in New Zealand, (now deceased). I would say it was added at a later date. If memory serves, GPC/Numrich in New York had reproductions made to Australian HT specification rather than No.4T which is distinctly different.
Brian