Frank;
More than likely those covers were nothing more than inspection covers that were located in the pressurized portion of the aircraft known as the "pressure vessel." As they would be frequently removed it would normal for them to be sealed as the fit between the covers and the floor structure would not be that precise. Additionally when an aircraft is pressurized it does expand like a balloon and the rubber or cork seal would be necessary to ensure that there is no leakage in that area. Hope this answers your questions.