Ball joint size

[Oil-burner]

I've had 2 cars come to me with the wrong size balljoint in them! Local parts places sell them the wrong size and they just put them in not knowing the difference!
That's fair enough if you've experienced it before but obviously the OP hasn't so trial and error is the best way!
What If the hubs have been swapped at some point? It's ok saying p1 16mm p3 18mm you can't be sure unless you know the cars history!