That rating is an Aviation Metalsmith (AM3/c WWII) or Aviation Structural Mechanic (ASM-3 modern rating). I would guess, based on the label in the uniform, that this is an early uniform as it appears to read "AM 3/c". I don't know when they changed the rating's name but you could research that. You can always search for WWII Navy Ratings and Badges to find lots of information and listings.
"Dungaree" refers to dark blue cotton denim used in a sailor's work trousers and worn with the light blue cotton "chambray" work shirt. When a sailor was wearing "dungaree's", he was in this work uniform.
There was also a dungaree jacket with a shawl style collar, 5 sew-on buttons (vice rivited metal buttons) and 2 patch pockets. Even though this was clearly a jacket and not a pull-over jumper, it was called a "dungaree jumper."
THe jacket you have pictured is not a dungaree jacket. It is a "Utility Jacket, N-3." It was a late war (early 1944) replacement for the Navy Combat/Utility Coat. The N-3 jacket was cut just like the USMC combat uniform jacket but had a different imprint on the metal buttons and was typically darker in color than the USMC jacket. While these were officially named "jackets" they were worn as shirts. There were N-3 shirts cut just as you would expect a shirt to be styled, placket front, patch pockets, etc., but of a lighter weight material. There were also N-3 trousers made of the same dark OD herringbone twill cloth as the jacket.