The first Marauder was indeed shipped in early 1975, though the model was introduced in 1974.
It was a difficult time for sales at Gibson, believe it or not.
There's a very good chance that your neck was originally produced in 1972, (ostensibly destined to be mounted on a Flying V), but sales were down, so the neck may have sat in bench stock for a couple of years.
I love the Marauder, personally.
I remember that KISS endorsed them, and I was a big KISS fan in those days.
During the Destroyer tour (1976 or so) Paul Stanley used to play a black Marauder on one particular song, and then would destroy it on stage, a la Pete Townsend.
Gibson shipped dozens of Marauders to KISS, for just that purpose.
:)