Rothana Heavy Engineering
The walker was commissioned by the Grand Army of the Republic when the clones were commissioned. ATAT300 was created by Rothana Heavy Engineering, as were the rest of the All Terrain Tactical Enforcers.
In 21 BBY, ATAT300 was stationed on board the Resolute. While on deployment to capture bounty hunter Cad Bane, they entered the system of Devaron. The cruiser had no boarding craft, so Jedi General Anakin Skywalker decided to improvise.
He called for Admiral Wulff Yularen to unlock ATAT300, Walker 773 and another walker so they could land on the Separatist cruiser. Captain Rex ordered Execute Battalion to take ATAT300, while Carnivore Battalion took Walker 773.
They boarded the ship, where they took out several droids on the hull of the ship. The battalions exited the their walkers and deployed into the ship, while the walkers managed to re-board the Resolute, as the cruiser began to explode.