You bought that rod from me? It is still with in the right size range but I buy hundreds of them at a time and thread the nut on by hand if it was too small I think I would have noticed it. Maybe not. The pic below is from the batch I bought a few months ago, that I have been selling since.
Either way the coupler should still fit. If not there are a ton of parametric ones on everything just make one a bit smaller on the 5/16 end. I have to thread the rod in on the couplers I sell before I put a screw in there. They are very snug.