I've seen 6's with the step at the back like that, but they were ex-South African military.
They were RHD from the factory but don't think they were that old, mid to late '60s I would have guessed.
I think they were also made in RHD for Israel, KSA & a few other frequently warring nations around the Med.
A 105mm (M40?) recoilless rifle used to be mounted in the back so the chassis should still have the additional stiffeners if it's not been hacked about too much.
Have a look here, the CJ 5/6 as they call it is the first vehicle mentioned.
http://www.saarmourmuseum.co.za/lesakeng.htmlEdit:
Also got a military tow hitch to seal the matter!