How do I specify an external at a revision? the external definition in my previous svn setup isn't working even after I change the url

your svn seems to add the latest version to the url instead of the one I specified.

svn: warning: W205011: Error handling externals definition for ‘Build/Tools/FlashImageHeader’:
svn: warning: W160013: ‘/scm/repo/XSeries/iris_pcapps/!svn/bc/765/code/sources/644/FlashImageHeader/trunk/bin’ path not found
At revision 6963.
in my external definition:
-r 644 /scm/repo/XSeries/iris_pcapps/code/sources/644/FlashImageHeader/trunk/bin Build/Tools/FlashImageHeader

I have tried all the various ways to specify the version and I see this error no matter what:
svn: warning: W170000: URL ‘https://001vl1sccmap01.zollmed.com:8443/scm/repo/XSeries/iris_pcapps/code/sources/FlashImageHeader/trunk/bin’ at revision 644 doesn’t exist
but I can get to that version in the gui.

I figured out how to get the external to work.
/scm/repo/XSeries/iris_pcapps/FlashImageHeader/trunk/bin@644 Build/Tools/FlashImageHeader