The boundaries of the City of Port Isabel shall be the same as they have heretofore been established and as they existed on the date of adoption of this Charter, and as established by that certain instrument dated March 23, 1928, filed on record in Volume 226, Page 66, of the Deed Records of Cameron County, Texas, and as the same may have been amended from time to time, and as further set out in Volume 7, Page 23, of the Map Records of Cameron County, Texas, and any additions thereto duly recorded in the Map Records.