This martin house adapter was origianlly designed by Del McKinnon with a round hole and a crescent. I expanded upon it and made it a round hole and a WDC Modified Excluder entrance. This plate offers the versatility of a round hole and a WDC Modified Excluder entrance. You can start with a round hole, and then, once you have birds, turn the plate over and have a WDC Modified Excluder entrance. The TW-24A sells for $3.75 each plus S&H.