The "Copyright" field in RPM spec files is obsolete. 
"License" should be used instead. 

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Can you provide a pointer that shows this obsoletion?

http://www.rpm.org/RPM-HOWTO/build.html#SPEC-FILE

still says Copyright.

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The rpm.org site is much more obsolete than this tag  
<wink>.  
  
Here is an excerpt from a message of Jeff Johnson in  
rpm-list (subject is "Re: three questions about building  
rpms"):  
  
----  
[...] 
This is historical legacy. Originally rpm had  
        Copyright: GPL  
but everyone said  
        GPL is not a copyright.  
  
So, rpm changed the tag name to License:, and, for  
backward compatibility, used the same numeric value as  
RPMTAG_COPYRIGHT. Now, everyone gets to ask the next  
question  
  
        Which is it Copyright: or License:?  
  
and the answer is <shrug> :-)  
----  
  
Every distribution working with rpms, including redhat,  
has changed (or is changing) the tag to License.  
Copyright, as Jeff said by himself, is a misgiven name  
for that field.  
 

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So what is the minimum version of the RPM software that
accepts the License: field? It is my understanding that
rpm(1) may blow up if it does not recognize a field.

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It appears that you need rpm 3.x, which was release 1999. I
think this is safe enough to accept this patch; applied as
bdist_rpm.py 1.30.