MacOS – US keyboard layout on macOS Sierra switches backtick to other key on external keyboard

I'm using a US keyboard layout on two Swiss (CH-de) hardware keyboards. On the builtin MacBook Air keyboard it works, while on an old external Apple keyboard (M7803) I've the problem that the key which usually produces a backtick (`, i.e. the one left to the z on a US keyboard) doesn't work, producing a § instead.

Showing the Keyboard Viewer displays the backtick in its proper place (clicking it with the mouse also works), but as soon as I type a character, the keyboard layout shown changes (having no key left of the z) and the backtick moves to the key left of the number 1. Actually pressing the key left to the 1 actually produces a backtick, so I guess I could get used to that…

But what's going on here? Why are the ` and § keys switched on the internal and external keyboards? The odd behaviour of the external keyboard started appearing only when I upgraded from El Capitan to Sierra.