Correct Ball Joint still not fitting - What am i missing?

Have just pulled out old upper ball joints on a 73 Duster 340. Ready to install a new PST upper ball joint (large size for MY73) And it will not fit. Seems to be about 1mm larger in diameter than the old one. In the picture the old one is on the left and new one is on the right. The diameter difference is in the threaded body not the nut which is the same on both. Ignore the socket that's for the small ball joint.

Any help will be appreciated.