Here is the January 23, 2002 letter from Caldera, allowing use, modification and distribution of Ancient UNIX, "including creating derived binary products created from the source code", as distinguised from System V. Do you see anything that says the code can't be used in GNU/Linux? The PDF is available here.
Dennis Ritchie's page on Computer History Links puts it in context:
"The current nominal proprietor of the source code for early research Unix systems (5th Edition through 32V) is named SCO. Today's SCO was previously called Caldera, Caldera having bought the rights from the earlier instance of SCO. This earlier SCO made them available for research and personal use. Until recently, Caldera did as well; now their offer has been withdrawn, presumably for reasons having to do with their suit against IBM. An earlier (2000) version of their license has evidently been withdrawn; this is a local copy. There was also a subsequent, less restrictive, license; its announcement is shown here in PDF format."
Of course, whether such an offer can be "withdrawn" is a question. The letter he is referring to as "less restrictive" is the one made available here now as text. You can also find the letter in TriBUG's mirror of The Unix Heritage Society's Ancient Unix archive. TriBUG is "the web site for the Triangle Area BSD Users Group (TriBUG), a users group for people in the Research Triangle Park, North Carolina area who use, advocate, or express interest in BSD and its current derivatives and incarnations. BSD, or Berkeley Software Distribution, is the name of a line of Berkeley Unix releases from the Computer Systems Research Group (CSRG) of the University of California at Berkeley from 1977 to 1995, and continued today through a variety of vendors and variations."
As you will recall from the article by Dr Stupid on The BSD Smokescreen, M. Warner Losh, of the FreeBSD core team says that while there is BSD code derived from Unix 6th and 7th edition, as well as 32V, all specifically allowed by this letter, there isn't any System V:
"There never was any System V code in any BSD. Ever. The IP claims that USL made its 1992 suit were based on the inclusion of sixth and seventh editions and 32V. While these were the forerunners to System V and System III code bases, they are not specifically System V or System III. Furthermore, SCO released, under its ancient unix program, all sources that predated System III and System V to be freely distributed under a BSD-like license. These specifically included 6th edition, 7th edition and 32V."
The letter reproduced as text here confirms what Mr. Losh remembered.
[address, phone, fax]
January 23, 2002
Dear UNIX® enthusiasts,
Caldera International, Inc. hereby grants a fee free license that includes the rights use, modify and distribute this named source code, including creating derived binary products created from the source code. The source code for which Caldera International, Inc. grants rights are limited to the following UNIX Operating Systems that operate on the 16-Bit PDP-11 CPU and early versions of the 32-Bit UNIX Operating System, with specific exclusion of UNIX System III and UNIX System V and successor operating systems:
32-bit 32V UNIX
16 bit UNIX Versions 1, 2, 3, 4, 5, 6, 7
Caldera International, Inc. makes no guarantees or commitments that any source code is available from Caldera International, Inc.
The following copyright notice applies to the source code files for which this license is granted.
Copyright (C) Caldera International Inc. 2001-2002. All rights reserved.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
Redistributions of source code and documentation must retain the above copyright notice, this list of conditions and the following disclaimer. Redistribution in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
All advertising materials mentioning features or use of this software must display the following acknowledgment:
This product includes software developed or owned by Caldera International, Inc.
Neither the name of Caldera International, Inc. nor the names of other contributors may be used to endorse or promote products derived from this software without specific prior written permission.
USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA INTERNATIONAL, INC. AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Very truly yours,
/signed/ Bill Broderick
Director, Licensing Services
*UNIX is a registered trademark of The Open Group in the US and other countries.