I didn't read all the posts prior to this, so I apologize if it's redundant. . .
The brass you have there is a Winchester. And as you suspect, it's a 1978. I started reloading in 1984, so I have a bunch of them (most of mine read "83," but I have others, including 78's.) Obviously, it's from a +P loading. It's good brass, for the record. I keep mine segregated and I use the +p headstamped brass just for +p loads. I don't know if the brass is any tougher than others (although they do seem to put up a little more fight in the press). But I use +p brass for +p loads just so I know they're a higher pressure loading. Also because I'm OCD lol.
If you ever find yourself in a fair fight, it's because your tactics suck." -- Allen West, quoting Marine 1st Sgt Jim Reinfinger