Use "2.1c1" since that is what Guido used for the Python version number.

Doc/Makefile

@@ -67,7 +67,7 @@ TOOLSDIR=	tools
 
 # This is the *documentation* release, and is used to construct the file
 # names of the downloadable tarballs.
-RELEASE=2.1rc1
+RELEASE=2.1c1
 
 PYTHON=	   python
 DVIPS=	   dvips -N0 -t $(PAPER)

Doc/texinputs/boilerplate.tex

@@ -6,5 +6,5 @@
 }
 
 \date{\today}			% XXX update before release!
-\release{2.1 rc 1}		% software release, not documentation
+\release{2.1c1}			% software release, not documentation
 \setshortversion{2.1}		% major.minor only for software