A member of the following classes shall be presumed to have been lawfully admitted for permanent residence even though a record of his admission cannot be found, except as otherwise provided in this section, unless he abandoned his lawful permanent resident status or subsequently lost that status by operation of law: (a) Prior to June 30, 1906. An alien who establishes that he entered the United States prior to June 30, 1906. (b) United States land borders. An alien who establishes that, while a citizen of Canada or Newfoundland, he entered the United States across the Canadian border prior to October 1, 1906; an alien who establishes that while a citizen of Mexico he entered the United States across the Mexican border prior to July 1, 1908; an alien who establishes that, while a citizen of Mexico, he entered the United States at the port of Presidio, Texas, prior to October 21, 1918, and an alien for whom a record of his actual admission to the United States does not exist but who establishes that he gained admission to the United States prior to July 1, 1924, pursuant to preexamination at a United States immigration station in Canada and that a record of such preexamination exists.