The car probably did not come with a power antenna. Those were on the higher end Becker radios. Blaupunkt radios had a manual antenna. Although we can't see the bottom, this photo looks like a manual antenna. The threads and nut that would fit on the outside of the fender are missing. The star shaped ring at the bottom would fit against the inside of the fender and the nut would tighten from outside. It's a simple installation but you need a new antenna. Unless you want a power antenna, you don't need a Hirschman.
If your radio has provision for a power antenna, it would mount with the bolts inside the fender. You can buy a new power Hirschman for under $100.
By the way, the Imageshack site you're using for photos is clunky and intrusive. You can post photos here without linking to an outside site. Search for "photo" and you'll find threads here with instructions.
Good luck.
Ray
'68 280SL 4-spd Coupe