The hidden ones are a hex head sheetmetal screw holding a large, formed steel "washer" to the rocker, that the aluminum panel slips down onto. If it is a Skylark or GS the holes will be in the rocker panel. The two end screws are rounded flathead stainless sheetmetal screws. Front screws into the fender lip and rear into the wheelwell lip. There will be pics in a 64 or 65 Buick assy manual.
