Drag Blocks are a component of downhole oil tools that are used to center the tool in the middle of the casing. Springs set behind the Drag Blocks keep the Drag Blocks constantly pressing outward with equal pressure, as the tool runs down the casing to the bottom of the well (or up) the Drag Blocks press out keep the tool perfectly centered and prevent damage to the tool.
The outside casing of the Drag Block has been specially heat treated to much harder than typical steel at 55 HRC. This drastically increases the durability and lifetime of the part without increasing the price of the drag block by using specialty strong steel.
The two notches seen on the top of the Drag Block are wear indicators. When the contact area of the part (the top) has worn down to the point that these notches are no longer visible, it is time to replace the part.