One of the mods that had to be done to all Number 5's was the fitting of safety chains. I have a 1960's era RAEME trailer andand 1970's Number 5 and both have safety chains fitted, by the army. So, on that basis, chains are "original".
Having said that, my first Number 5 which I sold over 10 years ago, and which I got direct from the army auctions, did not have chains.
That should make it easier for you to decide lol!