Review on Dometic 3109492.003 Polar White Lower/Upper Side Vent: Enhance Ventilation Efficiency! by Jamie Rogers

I have no idea where to get the cutout dimensions for this vent. For Dometic 3109492.003 the correct vent is 19 1/4" x 9 11/16" according to the "Dometic Vent Installation Instructions". In front of me is a vent being checked with a tape measure. Another issue is that the mounting holes are within the cutout size. Since you can't screw a screw in air, there must be some sort of fastening system that isn't provided by the vent. The lack of correct cutout dimensions, instructions, mounting system details should not mean "DO NOT BUY THIS".