I am no expert on this but I was looking at purchasing a '73 Thing once and it was supposed to have the 'original' motor and I did do some research on this topic.
An original '73 Thing fan shroud will have no fresh air vents in the back (as oriented with the car) AND it will have the 'doghouse' in the front for the oil cooler.
Since yours does not have the dog house, I would say it is not an elusive '73 Thing fan shroud.
But for that kind of money, I would get a second opinion. Like I said, I am no expert.