"The fire support vessel of the MRF was the Monitor. These had the same superstructure as the ATC, but were completely different forward.  The monitor had an open well deck, forward of the superstructure, that contained a Mk 2 Mod 0 Navy 81 mm mortar and two 7.62mm machine guns.  A rounded bow replaced the bow ramp of the ATC. The redesigned bow mounted a 40mm cannon (with a co-axial 50 caliber machine gun) enclosed in a turret. The 40mm was the main gun of the riverine forces and it provided a high volume of fire during landing operations. In addition, at least two Mark 18 grenade launchers were carried along with the individual weapons of the crewmen. Later, Program 5 Monitors replaced the 40mm turret with the 105mm turret of the Marine Corps LVT(H)-6 amphibious tractor (Amtrac)."

from www.warboats.org