I seemed to recall I once chanced across a website which lists Gibson Instruments by their serial number.
A quick web-search brought out these few sites which may help.
http://www.gibson.com/downloads/bluebook/GibsonSERIALNUMBERS.pdf
http://www.mandolincafe.com/archives/faq.html#gibsonserials
http://chrisbsmusicinc.stores.yahoo.com/gibsernum.html

QUACK!
Geoff the Duck.
p.s. I tried to resist saying you can identify a mandoline because it looks like a guitar that shrunk in the wash! (I couldn't!)