Another, rather speculative guess: what, if this symbolizes cutting a line, which would be the control key? (It doesn’t add up with anything like cutting line two of three, though.) Surely, you’d want a control key and I can’t see any on this keyboard.
We may even speculate that the second one would normally go to the extreme left position on the 3rd row, which is here taken by the “ß” key (where is shift-lock nowadays). This would be a common position for CTRL on earlier keyboards, on the left and right of row 3. It would also explain why “ß” is located so peculiarly outside the normal block of alpha-numeric keys.
(Backspace, which seems to be at SHIFT+“ß”, would have normally been where the upper of the two “Is-it-CTRL-or-not”-keys is located in this layout. This makes some sense to me: they had to find a key for “ß” and relocated the left CTRL key. It would have made even more sense, though, if they had just dropped the second key and kept backspace where it’s normally found. However, as a German keyboard user, you’re used to minor nuisances like this.*)
Edit: As I’m now rather convinced of this, sorry for side-tracking regarding the segment mark.
*) Fun trivia: guess where the UNIX home directory is on a German Mac keyboard – yes, it’s Option+n and it’s a dead key, so you have to press SPACE after this. And the square brackets are on Shift+Option+5 and 6 respectively and not marked on the keyboard, etc, etc. There’s a cost for having umlauts, ß and § (the latter being used to mark legal paragraphs)…