1st series vs. 2nd series

[Second Series]

Someone asked about hubcaps and the discussion turned to 1st Series Vs. 2nd Series designation. What is considered 1st series and 2nd series?
Is my 1 ton Chevy Panel Truck built in December of 1947 a Second Series?
Is Second Series only referring to 1955 trucks built after March, or is it a range of years 1955 to 1959?
Is First Series only referring to 1955 trucks built before April, or is it a range of years 1947 to 1955?
The AK series or Art Deco trucks were built from 1941 to 1947.
The Advance-Design trucks were built from June 1947 to March 1955.
The Task Force trucks were built from late 1955 to 1959.
The Chevrolet Parts and accessories catalog shows the 1946 series as First Design and Second Design.
It also shows the 1947 Second Design.
It shows the 1955 First Series and the 1955 Second Series.
It also shows a range of years 47-55 for First Series and 55-57 for Second Series.
An article in Hemming Motor News refers to a 1946 as a 1947 First series, and a ’54 as a 1955 First Series.
The LMC catalog lists a range of years 1947 -55 as First Series, and 1955 to ’59 as Second Series.