Sellior & Bellot aka S&B make their own brass, or at least RWS and S&B are made at the same plant in Europe.
I bought some factory S&B for my 300 some years back, the brass is tough, unfortunately, I moved house and one of the removalists must have reloaded cuz about 500 cases went missing in various calibres, luckily he didn't find the rest of my brass!
Cheers.