Rutishauser might have gotten it from Konrad Zuse's Plankalkül, which used the right double arrow ⇒, which looks a little like =.

Plankalkül had the order and the assignment reversed relative to C, so to increment a variable Z1, you'd write:

    | Z + 1 ⇒ Z
  V | 1       1
  S | 1·n 1·n 1·n

(The first line is the 'main line'; Z stands for Zwischenwert, i.e. "intermediate value". The second line is the 'value line', which contains the indices of the variables -- to store Z1 + 1 in a new variable Z2, you'd replace the second 1 with 2. The third line contains Struktur-Indizes.)

The discussion here show how much computer languages are human's work, with all humans' little approximations, taste, creativity, etc. Great read :-)

What are Strukturindizes?

The translation would be structure indexes.

Yeah, I think that would be clear even if I didn't speak German, which I do. I'm curious what their semantics are / what they're for.

Bauer and Woessner [1] weren't very clear on their semantics, but according to Bruines [2], they're type annotations.

[1] https://web.archive.org/web/20090220012346/http://delivery.a...

[2] http://www.cs.ru.nl/bachelorscripties/2010/Bram_Bruines___02...