In a balun I have, (model 50-7725 from CPC, datasheet -> http://www.farnell.com/datasheets/2365902.pdf?_ga=2.142463986.1590159842.1535980880-667311346.1535980880)
It shows the Balun as:
What is the see through circle on the signal line, to the left, which connects to ground, mean?