Sailflow is spot on. Remove the set screw, then pry the jaws open with a flat screw driver and small hammer until you can get a fine flat file in ther gap. Chuck in a vice and file the gap about 1/8" wider. Place on boom and tighten If it is snug, tighten and go sailing. If not, repeat the filing step.
I don't know why some of the old goosenecks were out of spec, but this is a common issue for many of them.