The documentation of csplain is on the site

   http://petr.olsak.net/csplain.html

The version of the csplain is in the format "Mon. Year".
It can be found in the file tex/csplain/base/csplain.ini
at the line 15 where the banner of csplain is defined.

More information is in the file tex/csplain/base/csplain.ini.
