It's mentioned in the HOWTO 

Forum: Xircom PCMCIA Ethernet Adapter Issues
Re: RE-100 not init on startup (David Hammerton)
Date: 2000, Jul 05
From: David Hinds <dhinds@pcmcia.sourceforge.org>

Move /etc/pcmcia/network.O to /etc/pcmcia/network, and read the PCMCIA-HOWTO notes about Red Hat. -- Dave

to: "It's mentioned in the HOWTO"