Victor, that disconnect switch looks just like the Hella which I used in my race car 15 years ago, and which I have seen in various Edmonton-area auto parts stores since then. Since Hella is an electrical
equipment manufacturer (best known for driving lights), they might
be the original maker. It is a solid and clever design which mounts easily in a panel. It was rated for (as I recall) 400 amps, to handle starter current. The plastic "key" handle did snap eventually, making it annoying to use now, but I believe that was due to use in cold (-20 C) weather.
In the race car I followed the standard practice of disconnecting the positive cable, which made sense to me. I'm not disputing anyone else's logic - that's just how it was done. It may be helpful to note that in a race car the object is to remove power from ignition and other circuits, while in other applications of battery
disconnection the object is to prevent accidental shock in contact with hot-side components.