Thanks for the education, I remembered that I had some spares in the garage.Those turnbuckles shown are the old style that were made by Navtek. They have not been manufactured for over 20 years and for good reason. The middle threaded portion fails without warning typically right inside of the square wrench flat center section. We lost a perfectly good J22 mast during a regatta when one of the cap shroud center sections let go. I also lost a center section on the inner cutter headstay on the family cruiser, a Pearson 365.
If it were me, I would change all of the standing rigging before I sailed the boat next as a failure will cost you the stick, and the stick will typically cost you the boat. The new chrome over bronze center body style turnbuckles are less than half the weight as well. The Navtec are stainless on stainless on stainless and are prone to thread galling which on a J-24 where you are (or should be) adjusting rig tension race by race, are likely to have a galling situation.
That striped situation you are referring to could just be galling a turn of so before it locks up forever. I can't help but notice the disparity in adjustment on the port cap shroud between the threads showing on the upper vs. the threads showing on the lower portion. . I am hoping that you did not just wind up the wire end to meet your required tension as the wire will not be happy with you.
That being said and life being the ultimate reality, we do have a box of used Navtec bits and pieces down in our rigging shop for that customer who needs one more season out of the old girl. Verify what the stud diameter is (my guess is 5/16"-24) and I will take a look to see what we have kicking around.
You may also want to consider installing those Velcro quick pins to keep your turnbuckle adjustments where you last left it. They are a wonderful KISS type of product where after the second time you use them will admit you should of had the years ago.
Obersheimer Sailing Supply