Hi Cliff,
It might be because the 240Z has a different pressureplate height than the later 280Z and 280ZX. A lot of people just simply buy the 280ZX with the collar and be done with it. I think that will be eventually the only option available, due the demand reasons. There is just a lot more 280Z and 280ZX's out there.
Your 240Z might already have had a replacement 280Z pressure plate when you changed it. If its still original, then you only need the collar from a 280Z or ZX and it will work perfectly.
The sketch and photo might help clear it up a bit. Im not ure if the sketch is 100% correct. I only know the 280Z, 280ZX non-turbo and the 2+2 & Turbo is correct because I have both in the garage somewhere. Btw, The photo is not mine, I think I found it here. Someone here might be the owner, so thanks