The class HTML files contain the build number (which was recently added):

java.lang.String.html
<!-- Generated by javadoc (build 1.4.2-rc) on Mon Jun 09 04:03:11 PDT 2003 -->

but the index.html page, for one, does not:

<!-- Generated by javadoc on Mon Jun 09 04:02:26 PDT 2003-->

Perhaps the build number should be added to the index.html page?
(Or is it deliberately missing because there is no version-dependent
API content? It shouldn't, as the version number is supposed to signify
the HTML as well as the API content.)