Everything is different.
The '55 first series (designated by GM as Advanced Design series) belongs to the 1948-early 1955 family.
The 1955 2nd series (designated by GM as the Task Force series) belongs to the Late 1955-1959 series trucks.
All this applies to the 1955 year model. There are no 1st or 2nd series trucks for 1956-1959.
The 1955 1st series and the 1955 second series are completely different designs and as far as I know don't share any body parts, gauges, dash or anything else except maybe some driveline parts. (engine, trans.)
You can go to the Gallery and take a look at a 1954 truck. This what the 1955 first series looks like, while the 1955 second series looks like a 1956.
All this is only because GM changed truck designs in the middle of the year in 1955 and they came out with the new Task Force design.
