2.11BSD is patch level 431, dated April 2000.

ftp://minnie.tuhs.org/UnixArchive/PDP-11/Trees/2.11BSD/usr/src/new/
    or
http://minnie.tuhs.org/Archive/PDP-11/Trees/2.11BSD/usr/src/new/
    or
http://minnie.tuhs.org/cgi-bin/utree.pl?file=2.11BSD/src/new
