These patches are described in:
        ftp://freebsd.org/pub/CERT/advisories/FreeBSD-SA-97:05.open

They are meant for:
open-patch.a            FreeBSD-current before 1997/10/23
open-patch.b            FreeBSD-current before 1997/04/14
open-patch.2.x          FreeBSD 2.1.*, 2.2.*
                        FreeBSD-stable before 1997/10/24
                        FreeBSD 2.1-stable before 1997/10/29