I don't know the answer, but looks like sequence numbers to me. But this may bo totally wrong. For what you want to do, this thread may be of interest for you, especially Grant's ideas: https://lists.netfilter.org/pipermail/netfilter/2005-May/060299.html
You may want to take a look at the newer version of this script. It has grown a LOT. You can find the thread in the mail list archives here: https://lists.netfilter.org/pipermail/netfilter/2005-June/061362.html
