TJ, the production of all model year trucks started in the previous calender year, and back then vehicles were usually titled as the year of first sale, not the model year - a '56 truck built in '55 would not be unusual, but one built with
previous series parts would be - so if the pic Tim links is your purchase, as he said, it's a
Blue Chip series GMC, 55-57 .... but what you've really got is a basket case, and no telling what has been done to it over the last 50+ years to keep it together ... it may even have been 'upgraded' with a Chev radiator
Bill
