It's a legit Heuer watch as the model 756, and is the female Monnin....which is why it's a tiny 28mm of a thing.
If you've read anything about the Monnin watches and how they came to market, then this is exactly the same for the ladies version in around 1979.
Originally they came with a red/black bezel, cathedral hands and were made in France. Later production was done by Heuer so were Swiss made went to Mercedes hands and the bezel became all black.
There's no reason to think a black bezel would not have left the factory on a Monnin produced one as it may have been a transitional piece or towards the back end of the run. The hands are correct as far as I can see as are the lugs and it is in my opinion original and clearly in great condition.
Here's some pics and story by Jarl;
http://classicheuers.blogspot.ae/2010/06/one-for-ladies-ref-756.html?m=1
Ladies watches are in a different league of value, as there simply isn't the demand, and female collectors may also find it a little small these days preferring the bigger versions.
I'd think there maybe money to be on it though, as it's a Monnin produced Heuer.