Part number 18032 stud for the Atlas is not the usual "stud", there being a hex section between the coarse and fine threads. One tightens it into the head using a ring or open end spanner.
If you wish to retain that originality, your course is set ..... The long end with the coarse thread goes into the head. You will need the proper 3/8 British thread helicoil and tap drill ..... probably not easy to find.
If you wish to make your own stud, I would helicoil for a 3/8-16 NC. As Deets55 says, the coarse thread is in the aluminum, the fine thread (3/8-24) for the grip end. If you go this route, you will need a stack of washers, or a ring spacer, to make up for the missing hex section, otherwise the head steady brace will not properly align on the head.
Slick
Edit: The thickness of the hex section is about 3/8 inch, as best I can "eyeball" it without removing the brace.