It was 56 and had a Y serial. There are some ways to tell a half built (Your under turn signal trim is 55 as you know) I think the voltage regulator is on the firewall on 55 and the inner fender on 56, or visa versa, also I believe 56 had a "hand clearance", wide, dimple on the cross bar so as to get better access to the hood release. The 55 was straight. As Red58 said, they were often titled in a different year. There are "half built" trucks and there are pure trucks. I have only seen this in 55-56, possibly because of the late release of the 55.2 and they wanted to hurry up and switch to new engines and all 12V for parts commonality. I am not sure about 3/4 ton models. Many times, in dating parts, a stamp showing previous year is proof that it is for the following year because they made the parts, and vendors made the parts, in time for the build and sales in Sept/Oct of the preceding year of the model year.
