On the Camels it was a purpose made fabrication welded to the rear cross-member to take, from memory, a Dixon Bate pin hitch.
If you speak to Mark at Discoparts, he'll sell you a mounting plate for his HD bumper which is very similar in design, except it bolts on. He'll make you one without the square hole too. You can then drill this to suit whatever hitch you'd like to fit and trim your existing bumper around it.
