Cdplayer - I do not think that is quite right. The upper nut which has hard to distinguish flat sides and will accommodate a wrench is to secure the mast. Below it is another large nut which holds the antenna to the fender. A 25/32" wrench will work but the proper size is someplace around 18-20mm and I do not have one. I have not worked with it for a while but I remember that the antenna, mast and all will come down through the hole in the fender if you remove the 25/32" nut. On my 81 300SD the inside fitting to fit the curve of the fender is rubber or plastic or maybe originally it was something like plumber's putty but it provides a snug fit when the nut is tightened.

You may want to unscrew the antenna lead before you drop the Hirschman. I will try to post a couple of pictures - one inside and one outside showing the nut.