This envelope was given to then Major Richard H Morton when he was S-2 Intel officer for 1st Brigade, 101st Airborne 1966-67 and he was boss of the long-range reconnaissance patrol team. He told me the team came up with the earring and it sounds like he was not a big fan of it, but they gave him his own, perhaps in thanks for his tolerance.
I searched on line and couldn't find anything about how common this practice was. Most google search results for LURP gold earring are in reference to the book Dispatches by Michael Herr which has this passage about a LRRP soldier:
"I knew one 4th Division Lurp who took his pills by the fistful, downs from the left pocket of his tiger suit and ups from the right, one to cut the trail for him and the other to send him down it. He told me that they cooled things out just right for him, that he could see that old jungle at night like he was looking at it through a starlight scope. "They sure give you the range," he said...He wore a gold earring and a headband torn from a piece of camouflage parachute material, and since nobody was about to tell him to get his hair cut it fell below his shoulders, covering a thick purple scar. Even at division he never went anywhere without at least a .45 and a knife, and he thought I was a freak because I wouldn't carry a weapon."