The best reference I could come up with was a reference to someone who thought he'd seen a Berkley reference in the license to NT 3.51. That's not very strong evidence unless I can actually run across a copy of NT 3.51 and see the license myself.
The winsock.h headers *do* include the following:
* This file includes parts which are Copyright (c) 1982-1986 Regents
* of the University of California. All rights reserved. The
* Berkeley Software License Agreement specifies the terms and
* conditions for redistribution.
but as noted, this doesn't necessarily have anything to do with the actual TCP/IP stack implementation.