I recently replaced my 2 year old 50 gallon oil hot water heater (switched over to gas). I told the installers to leave my hot water heater since it's almost brand new and I figured I could sell it. When looking at it today, I noticed a strange hole in the side (see pictures). Does anyone know what this is? Not sure if it's always been there, or if it's something the installers damaged when replacing it. Any help would be appreciated. Thanks!