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I don't believe that the naming convention of cloned menus is actually documented anywhere (it's certainly not documented in menu(n) of tk-8.3.5 as packaged by RedHat.  The only promise that the documentation makes is that the clone "is a child of the original". (but that can be violated if "menu clone" is invoked directly:

% menu .m
.m
% .m clone .n
%
)

That said, the convention for automatically generated clones hasn't varied in the versions of Tk I'm aware of (at least 8.2 through 8.4) so it might be reasonable to depend on this.

Does anybody in Python development also keep up with Tcl/Tk development?  It might be reasonable to ask them to document the format of automatically generated clone names.