I think its post 1951.
The Marine Corps Good Conduct Medal was first issued in 1896 and was originally a ribbon and medal suspended from a clasp bearing the words "U.S. Marine Corps".
The clasp was eliminated after 1935 and the medal has remained unchanged in appearance since that time.
Enlistment bars, showing each honorable period of service, were used until 1953 when the Marine Corps adopted service stars to denote additional awards of the Good Conduct Medal. Also from its inception in 1896 until World War II, the medal was hand engraved with the name of the recipient on the reverse (except for during World War I when they were numbered on the rim instead). This practice was changed to stamping of the individual's name during WW2 and was done away with all together around 1951.
It is affectionately nicknamed, "The Good Cookie" by Marines
