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                        |  | 
                            1/4" diameter                  1/2" straight line centers                  4 holes per sq. inch                  20% open area                  1/8" & 1/4" thick                  |  
                        |  | 
                            3/8" diameter                  9/16" staggered centers                  4 holes per sq. inch                  40% open area                  3/16" thick                |  
                        |  |   
                            1/2" diameter                  11/16" staggered centers                  2.45 holes per sq. inch                  48% open area                  1/4" thick                |  |