There is a tool class called a "metal nibblers". Some are manual (usually for up to 0.035 in thick steel), others are compressed air operated (up to 0.062 in thick steel, just a guess). For the rectangular shaped holes with the quarter round corners, you would drill a center hole to fit the nibbler, drill the corners, and use the nibbler to get the rest of the material removed. Holes like what you are dealing with, are punched by a single die on a 30 ton stamping press.
Jim