The small screw at the top of the picture is the one , it screws into the base of the fluid reservoir and stops it coming off accidentally when you remove the three top screws!
The problem is that water or whatever wicks in between the reservoir and the master cylinder and sits in the well above the O ring, eventually corroding it enough that it starts to leak.
But by the time you have cleaned off all the corrosion and pitting there is a real possibility/ probability that the O ring wont fit tight enough to form a reliable seal.
The rest of the master cylinder is usually close to worn out by the time this happens anyway, so trying to bodge it doesnt really make a lot of sense, apart from the safety angle, and if you order a 12mm as a replacement you will have 20/25% more breaking power as a bonus!