As it turns out it was a case of having the same shortcut assigned to some Filter
But when I was adding it for the custom_open_command10 it was not shown to me in the box of
Tools > Options > General > Keyboard > Edit > Shortcut key currently used by
of it; as to which attest my point #3 in this topic viewtopic.php?t=13729 - i.e. I would had spotted this other assignment, especially as that Filter of mine has a visually profound name of
●●● f i l e s
So some glitch made my old shortcut not appear when I was adding it for the new feature of viewtopic.php?t=13720