The older Jacksons were 5/16x14, but the newer ones are not. A 5/16x14 shaft will not fit them. I have a bunch of shafts with that pin, but they won't work.
They've changes several times over the years. If you call jackson and describe the pin they can tell you exactly which one it is. They have used 5/16x14, 5/16x18, and 9/32x18 as far as I know.
I have owned (3 so far) have been a flat faced 5/16 x 14. Talk to Cookie to confirm as he modified about 4 of my standard shafts to fit. The first 1/4" or so has to be flattened to fit. Without that no 5/16 x 14 will fit. Hope that helps. I think I still have one of my shafts if you want to see if it fits. Call me if you want me to bring it in one day.