The letters in Lohe Qurani are also called "Huroof-e-Muqattaah" which means abbreviated letters at the beginning of chapters of "Quran-e-Kareem". Theses letters in the beginning of the chapters in "Quran-e-Kareem" are "Rumooz" means codes, ALLAH Subhanahu WataAla is the only One who has the knowledge of their meanings. We the muslims believe in whatsoever their meanings are. People should not pursue or act to know the meanings of these "Romooz" or codes.
As far as Our Beloved Prophet (صلى الله عليه وسلم) (PBUH) is concerned. He knew their meanings or not. The scholars or Ulama say that may be the Prophet (صلى الله عليه وسلم) (PBUH) knew the meanings of these "Romooz" or codes written in "Lohe Quran" whereas He was stopped to reveal the meanings to the people.