Prior to the publication of the June 1945, Manual of Military Occupational Specialties NAVMC 1008-PD, the Marine Corps generally followed the Army's system for classifying jobs for personnel. As stated in the manual, the Marine Corps often used Army publications  for determining military SSN's . After the publication of this manual, it would be the sole reference for classifying military personnel in the Marine Corps. The Army Manual, TM-12 426 would still be used for civilian personnel.
List of Vietnam War Era USMC Military Occupational Specialty Codes (MOS Codes), verified from information included in the Defense Casualty Analysis Extract File (DCAS), at the National Archives.
Here are a couple of tools to make finding a yearbook a little bit easier. If you've been diligently searching every day on the the most common sources, then why not let them automate the process for you.
The Keel is the Great Lakes Naval Staion Traing Center's Yearbook, known more accuratly as a "Cruise Book". If you didn't order one while in training, your chances of obtaining one now are slim, since a limited number of books were printed, basically only enough to cover the graduating class for that time period. Many are available to purchase online through various sources - mainly old book sellers and other retailers such as eBay and Amazon.