I have had one very similar to that on my car for just about the entire 10 years I've owned it. The knob is red on the one I have. It was a DMCX product way back when. I don't see it on their website at the moment though.
I found getting it onto the negative terminal (as mentioned in the instructions and as you noted) a pain in the butt because of the battery compartment on our cars. It's manageable to get it installed on the negative terminal, but it wasn't very easily reachable when you went to use it to disconnect the battery. That'd be ok in a planned situation, but not so much if you had an electrical fire perhaps and needed it off ASAP.
I have had mine installed on the positive terminal because of that the whole time. I've never had any issues with it there and it's easier to access. No electrical fires to speak of thankfully though either. Just wanted to mention that as I was concerned initially as to whether it would be okay to install it on the positive terminal (and against the directions it came with).