I received a question about the Free Frank privilege for military personnel in Vietnam. Here is information from the Department of the Army provided to new arrivals in Vietnam:
Free mail service went into effect 1 September 1965 for all military personnel serving in Vietnam and adjacent waters. Personal letters, letter size tape recordings and post cards in their generally accepted form are entitled to free transit. Letters sent under this free mail policy will be handled as US air mail. Your full military address to include name, rank, and service number must appear on the envelope. The word FREE in the upper right hand corner must be in YOUR handwriting. It cannot be typed or mechanically marked.
Mail may be sent free to the Foreign countries. Letters so mailed cannot be registered. They may be sent special delivery if an additional special delivery charge is paid. The words "Postage Paid" or "Port Paye" must be stamped in the upper right hand corner. APO will perform this service. Handwritten or type written lettering is not acceptable.
Packages under 5 pounds sent regular mail will, whenever possible, be handled via Air Mail between Saigon and the San Francisco Post Office.