No licensed and registered dentist shall designate in any manner that he has limited his practice to one of the specialty areas of dentistry expressly approved by the American Dental Association unless such dentist has completed two years of advanced or postgraduate education in the area of such specialty and has notified the Dental Commission of such limitation of practice. Nothing contained herein shall prohibit any licensed and registered dentist who has limited his practice prior to May 8, 1975, from continuing to designate such limitation.