Unbound by default would be sufficient.
Altap Salamander with german keyboard has this keys on the 2 keys left from Backspace key. As far as I can see keycode 219 and keycode 221. Though, on pictures of english keyboard layouts the square brackets are 1 row below. Anyway, I would assign this two keys (Ctrl+ß, Ctrl+´) because I'm already familiar with them.