Can't speak for the source trees them themselves more than I have already, as they came from all over - BL Research, USL, IBM itself, ISC, LCC, UCB, CMU, MIT, etc...; but IBM negotiated a very extensive SVR3 license with AT&T and and marketed it primarily an System V based system with "BSD & IBM specfic enhancements".
This license agreement was the basis for AIX shipments as well as all OSF shipments. While other places covered it, if you google around for articles from "Unigram/X"(Maureen O'Garia's crew) in the years of 1990-2000 in particular you should find a number of articles that described the transactions and how it effected the "Unix Industry."