To back up your PIX current license you actually need to look at the running activation key that can clearly be seen in at the end of the output of "show version".By simply saving this output you can also save the serial number of the pix as well as observe feature licenses that are currently enabled on the PIX.
The information you will see in show ver will be similar to this.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...