It's Warner Bros. from the 40s. I think a lot of that doesn't survive, which might include this score. WB started saving mono backups of scores more in the 50s, I think. On their amazing Waxman set from a couple years ago (which was licensed from Turner Entertainment/Warner Bros.), Intrada actually included only a single Warner Bros. score (in mono)...from 1961! (It was the most recent score on the set.)
http://store.intrada.com/s.nl/it.A/id.11293/.fEven the 50s score on the set was MGM, and the rest of the scores (which included one from RKO) were all 30s and 40s yes, but not Warner Bros.
Yavar