mos occ field 35 motor transport motor t marines training and job marine corps detachment fort leonard motor t supports battalion 1st marine roles in the corps motor transport motor t in the marine corps motor transport operator 3531 mos usmc auto mechanic marine corps job marine corps detachment fort leonard usmc mos 3530 to 3537 marine corps motor t mos mos occ field 35 motor transport marine corps motor t mos 3521 3531