There were no lance corporals in the Marine Corps when you served. The rank, which falls between private first class and corporal, has been around since the 1830s when the Corps also had lance-sergeants. It went out of use sometime after 1930 and was reinstituted in 1958. Reference: "Handbook for Marine NCOs" by Col Robert D. Heinl, USMC (Ret)
According to the 1979 edition of the handbook, the term "lance corporal" results from a marriage of the French word lancepesade and corporal. Lancepesade means "broken lance." Therefore, the term lance corporal can be translated as "an old soldier who has broken many a lance in combat."