5/16-14 Acme, like a JW? I didn't know Scruggs made any of those.
Anyway, I'm not a machinist but I have an idea that might be worth exploring. I believe if you take a regular 5/16-14 joint protector and chased the threads out several times to make them sloppy, I believe you could get them to work. I know you wouldn't want to do that with a shaft...but it would work well enough to serve its purpose for a joint protector.