The arc troopers are considered phase 1.5 due to their helmets being slightly different than both phase 1 and 2. Eventually, they all upgrade to proper phase 2 armour later in the war, but what you see in the senate murders episode is actually phase 1.5 arc trooper armour, which I believe was being proposed at the time of that episode they then gave the phase 1.5 to arc troopers and used there. The successor of the Phase I clone trooper armor, Phase II was adopted by nearly all clone soldiers of the Galactic Republic by the final days of the Clone Wars.
I found this information by comparing the appearances list of the armor in Wookieepedia to the chronological list of the Clone Wars episodes. Clone Cadets appears to be the first episode (in timeline) showing the Phase II clone armor, worn by Commander Colt of Rancor Battalion. Republic clone troopers are shown wearing two different sets of armor in the Star Wars prequel trilogy.
Their successors, the recruited Imperial stormtroopers, use the same armor design for decades, so why do clone troopers go through two phases of armor throughout a three-year war, and what's the. The first scene of transition armor was in the episode Senate Murders, which had a brief hologram of a proto-Phase 2 Clone. The next step was the Arc Troopers from Season 3, which are a sort of 1.5.
At the mid point in the Clone Wars, The Clone Army began the long and tedious process of upgrading their armor set from the Phase I armor to the Phase II armor. The first appearance of a phase 2 trooper (not counting ARC troopers) was in Season 4 Episode 1: Water War in a blink and you'll miss it scene. Season 4 Episode 5: Mercy Mission was the first prominent appearance (and also was produced before Water War, so is technically their first appearance).
Phase II Clone Trooper Armor was the commonly-used armor worn by clone troopers, after the start of the second half of the Clone Wars. The helmet design and body armor of Phase II armor differed greatly from those of Phase I armor. It was also highly improved, compared to Phase I Clone Trooper Armor, as it had installed controls and communications, as well as an advanced vision within the.
Throughout the saga films and the animated Clone Wars series, we see several variants of clone armor which are typically divided into two segments, commonly called Phase 1 and Phase 2. The Clone Wars Episode 12 shows us why Captain Rex combined Phase 1 Clone Armor with Phase 2 armor, to create the most unique armor of any Clone Trooper in the Grand Army of the Republic.