BioEditExperimental Phase 1 Armor (or simply ARC Armor or Phase 1 ARC Armor) was an early version of the first phase of Clone Trooper Armor.
Originally designed by Jango Fett, the armor was merely modified from original Mandalorian Death-Watch and Mandalorian Warrior or "True Mandalorian" armor. The only difference between the original Mandalorian armor (early Mandalorian bounty hunters, militia, and Death Watch members also wore kamas), was the helmet (of course) which was designed after the original government guards on Mandalore; the belt, and torso chest-plate were also different as well. The armor itself, is basically an early version of Phase 1 armor. (Hence it being experimental, and demonstrated from the ARC Troopers.)
Very few Phase 1 ARC Lieutenants wore this version of Phase 1 Armor during the battle of Muunilist.
The Null Class of ARC Troopers were the very first Clones to ever be Cloned years before the Clone Wars began. There were only 12 which were Cloned, and many extra sets of armor were left for the Alpha ARC's, to whom they wore modified Experimental-Phase 1 armor.
The Null ARC's were also named after the older Mandalorian Militia warriors who trained the Null ARC's. (Before Jango betrayed the old Deathwatch.)