Harry, I have never seen a restricted opening piano hinge before.
no, the ones I'm looking for are usually found on briefcases.
You know the front catches on tool boxes where you release the tension on a lower latch and then the latch rises and clears the catch on the top lid?. Well these hinges work on the same principle. When closed everything is flush, but as the lid is opened the little brass arms move out to 45 degrees and stop the lid falling all the way back.
Its very frustrating, I have a perfect picture of them in my mind, I know they exist, but after 15 minutes of google images I still cant find the little devils.