As stated, you are supposed to break/grind off the tab when mounting pots without an index hole. Tabs made from cast metal usually pop right off when you give them a tweak with a pair of needlenose pliers. Tabs made from sheet metal, however, need to be bent to the side, or ground.
Concerning washers and nuts, I have always found it frustrating that most pots only come with one nut, when you need two in order to adjust the height properly. Depending on the thickness of the material the pot will be mounted to, it is sometimes acceptable to just use one nut, one washer, and one lock washer. The pot just sticks up a bit high.