Help decoding grandfather's Military Duty & Unit

Attempting to trace my grandfathers' footsteps in the war. My paternal grandfather served as a PFC in the USMC, and passed before I was born. I have very little knowledge of his war experience

I need to scan all of the paperwork into pdf form, before it disintegrates, but I'm having trouble decoding my grandfather's separation paperwork.

It lists his Principal Military Duty as "Hvy Arty. F.C. man 724"

And I can't find the job number 724 anywhere. Does anyone know what this stands for in total?

I also have a picture of his unit somewhere in the South Pacific, and one of the marines is holding a sign that says"FMF, 3DB A". This, so far, is the only hint I have of his unit. I know FMF stands for Fleet Marine Force. And the 3 could possible be a 9, but does anyone know what DB A stands for?

 

Heavy Artillery, Fire Control Man. 724 is his Military Occupational Specialty. He was either 3d Marine Defense Battalion or 9th Marine Defense Battalion, probably A Battery. See:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1017.8525&rep=rep1&type=pdf
HyprWar: "Condition Red: Marine Defense Battalions in World War II"

Hope that helps. Semper Fi!