You'd have to be a little insane to have examine barrel production more than I have...
I was talking about finishers, not blank suppliers, - Ruhrstahl was both but they made a huge number of finished barrels, mostly for Gustloff. (Ruhrstahl was not a single facility, is was a massive conglomerate, bigger than almost any we deal with, blanks were made at one facility, finished at another... but they made a large number of things. They were the only company to do both, make their own blanks and finish them)
They were probably both, one of the largest blank providers (if not the largest - Deutsche Edelstahlwerke AG was also a part of Ruhrstahl) and also one of the largest finishers (probably not the largest though). Mostly they supplied Gustloff, they were contracted in 1938 to do just that, and ultimately they supplied most of their barrels. They also supplied SDP in 1943 and Brno in 1944/45.