This module is contained in the mod_tls.C file for ProFTPD 1.3.X, TLS_SERVER_M_SERIAL: Server certificate serial number:
HOWTO : Create a FTP server with user access (proftpd) There's some support for this guide in the hoary section
Most servers using Proftp are still vulnerable to attacks as they're using older versions of the software.
ProFTPD module mod_tls – IRIS Consortium
ProFTPD Logins and Authentication – Howtoforge