I'm crappy with this computer stuff, but if you go to a thread on page 3 of the general truck discussion you will find a thread "serial numbers", on page one of the thread is a response from tclederman, with a link for "that are unique to 55 or to 56 Blue Chip GMC trucks", check the booklet on the 56s on the left of the page, go to page 13 and it shows what you might be missing
