dilled potato and pickled cucumber salad

INGREDIENTS

• 6 tablespoons distilled white vinegar
• 4 teaspoons coarse kosher salt
• 2 1-pound English hothouse cucumbers, very thinly sliced
• 1/2 cup plus 3 tablespoons chopped fresh dill
• 3 1/4 pounds Yukon Gold potatoes (about 10 medium), unpeeled
• Additional coarse kosher salt
• 1 cup very thinly sliced white onion
• 8 radishes, trimmed, thinly sliced
• scant 3/4 cup mayonnaise (I get by with a bit less)
• Small radishes with green tops