Keeps out moisture and corrosion when antenna is removed. Stud mount plug threads clockwise into stud mount when antenna is not in place. If you live in a climate with a lot of rain, snow, sleet and hail, you may want to keep a couple of these in your glove box. When your antenna isn't in the mount you can screw one of these plugs in an keep the "nasty's" from making a new home. Also great for those who take their vehicle though the carwash or tend to back up a little too much when launching their boat (Ooops!). Fits any 3/8"-24 threaded mount or accessory. Additional Model #'s 221-PLG-1 221-PLG1 PLG-1 PLG1 UPC: 716414200805