That is present on all Abrams tanks. It is the lock down for the loader's hatch when the hatch is opened. The loader's hatch has two positions, fully closed and fully opened. For some reason, modelers like to display the loader's hatch in various angles. The hatch is about as heavy as a manhole cover. You don't want that thing bouncing up and down; it would smack you on the head and give you a concussion or slam shut and cut your fingers off.
Those six circles are part of a never used (and eventually abandoned) quick ammo replacing system. Instead of manually loading 50-some odd tank rounds through the loader's hatch, the crew could just hook up the complete removable ammo rack with a crane and replace it with a full rack. We called the rings "anti-sleeping bag mines" since that used to be a prime spot to sleep at night, but the rings made it not so.