Wow! You're incredibly lucky the whole rig didn't come down!
Those are the old, original Navtec style turnbuckles, with the harps on the ends and a double-ended screw stud in the middle, popular in the 70's. Newer, modern turnbuckles have a single open harp in the center and threaded studs on the ends. The Navtecs may still be available from Rigging Only:
http://www.riggingonly.com/stdrig04.htm#TURNBUCKLESI have
never seen one fail this way! Usually they fail at the lower swage, or (for the shrouds) the chainplate breaks below deck due to hidden crevice corrosion. I suspect this one may have experienced some abnormal lateral stress in the past, causing a fatigue crack to start in the stud.
Because this one broke, I would critically examine the rest of them with a magnifier. Pay particular attention to the upper shrouds, as they have some of the highest loading and have the least redundancy.
I just replaced all my standing rigging, and have the old turnbuckle bodies. I'd be happy to send you the backstay one, but I strongly advise you to inspect all the remaining ones, and the chainplates, and institute a schedule to replace them. You can do them one or two at a time (with the mast up) to spread out the financial hit over a few seasons, or bite the bullet and pull the mast and do them all at once as I did.
When reassembling a turnbuckle, be
sure to use an appropriate thread lube, such as lanolin (natural) or Tef-Gel (synthetic) on the threads or they will gall-up and jam when tightening.