I inherited an FG-150 (green label) from my uncle and found a site that, based on the serial number, dated it to Oct ‘66 “Made in Japan by Tenryu/Wada Factory”
Serial Number: 555787 K
Date code: 41._96 (seems like it’s missing a number in the middle, hence the _)
Old style “Reinforced Neck” truss rod cover.
Site is here if anyone wants to try it: