Four nearly identical versions of an ancient printed book, the Song of Enlightenment with Commentaries by Buddhist Monk Nammyeong Cheon (南明泉和尙頌證道歌), were examined by image analysis and comparisons to identify whether they are the identical versions or different versions in terms of printing techniques. Two out of four versions have been designated as Korean treasures and the other two versions are currently being examined for designation as Korean cultural properties. One of two Korean treasures has been spotlighted as a potential movable metal type printing book prior to the Jikji printed in 1377, as recognized by the UNESCO Memory of the World program. Heated debates over the printing techniques and printing dates have overwhelmed Korean historians for more than 50 years. Due to the subjective nature of the evaluation, it was hard to reach a unanimous decision. Finding objective new evidence is needed to end this heated debate. We found very clear evidence showing that one version is significantly different from the others and was likely printed using movable metal type in September 1239. It is the oldest extant book printed using metal type, 138 years prior to the printing date of Jikji.