Arne
Thanks for your input. I have upper and lower luff hauling parrel, and no fixed batten parrels - as in this photograph.
(This is not my sail but a similar Sunbird sail, of the same period).
I use the luff parrels to pull the sail aft when sailing with the wind forward of the beam, and to allow more balance sailing downwind. I have no problem with the hinges reversing. However, on starboard tack, the sail stands well away from the mast, having no fixed parrels, except one on the boom - it is retained only by the parrels at the luff and sheets at the leech.
I wonder why this type of Sunbird rig has no fixed parrels, and merely speculate that potential for hinge reversal when downwind, due to pull from a fixed parrel near the middle of the batten, may be the reason. Perhaps I should consult Robin Blain or Alan Boswell.