About the last place you want to anyone with any of them is in the head. Unless of course, you want to increase the chance of killing them. Same goes for a good punch for that matter.
Like anything else, there are places on the body you want to direct your blows for the desired effect. Just flailing away, isnt going to do you much good, beyond luck, good or bad.
A lot of the "brass" knucks you see these days, are actually made of steel and plated or painted. I got a couple of sets for cheap out of China a while back for something like $10/pair. They look to be those made as belt buckles, just minus the "stud" you see on them to hold the belt. Just as functional as the old brass knucks.
I also have a couple of the old "blackjacks" or slappers. Both the flat and round spring type. Again, the head really isnt the target you want to go for with these, they are meant for joints and other tender points on the body to help you get compliance/control.
The flat, "slapper" type are a bit easier to carry and a bit more versatile, as you have different striking options. You get more damage if you use the side/edge of them as opposed to the wide flat.
In the past, we used to fill a piece of 3/4" copper pipe with a cap soldered onto one end with lead and tap a cap on the other. It was just long enough to fit in your palm, and about the size of a roll of quarters or nickels, but weighed a good bit more. At least it didnt break open like the latter when you punched something. Still, if you want to hurt your hand, this is the best way of the bunch to do it.
I think these days, the expandable batons have taken the place of a lot of these type things, and give you better options, especially if youve had some stick training.
Another thing to consider too is one of the well made autos, assisted openers, or butterflies, which can be used closed as a kubaton/yarwa stick of sorts, and still be instantly, to quickly opened if you want something more.
What Im not understanding as far as the legality thing goes is, why would you be caught with any of them on your person, if you just used them on someone and its after the fact and going to be a problem? Why are you even still there?