Reverting to an old version of libstdc++6

[Joe Riel <joer@san.rr.com>]

This weekend I upgraded, and libstdc++6 went from
4.5.2-4 -> 4.6.0-2.   This has broken a commercial 
package I use; I now get 

symbol lookup error: /usr/lib/libstdc++.so.6: undefined symbol:
_ZNSt14error_categoryD2Ev, version GLIBCXX_3.4.15

I have a few simple questions.
(1) how do I determine what version of libstdc++6
that the previous debian version of that package was using?
That, is, the current package provides the file
/usr/lib/libstdc++.so.6.0.15 and a link
/usr/lib/libstdc++.so.6 -> libstdc++.so.6.0.15.
I assume the previous debian version did the same,
but with some other file, maybe libstdc++.so.6.0.14?
How do I find out what that file was?

(2) How do I locate and install
the previous version from a debian repository?

-- 
Joe Riel