Walkin Man. The x's in the sn are for your protection. Say you wrote in about a Colt single action, SN.# 123456 with a detailed description of the gun.. I can say, " Hey! That was the gun I had stolen from me a few years ago." If you'd sent in 123xxx, then I could not do this. That also gives enough info to determine the year the gun was made.
I hope that answers your question.