The Mk IV was dated by a helpful person on another forum about a year or so ago. If I can find the thread I will link it. Otherwise, I do not know how to date it. It does have "War Finish" on it.
Attachment 47940
I was unsure on the MkVI of the date. I did not know if the 1917 related to the year of the patent or the year of the arm's manufacture. Thank you for clearing that up.