I believe FMFM 1-3B "Sniping" is what you seek. As far as I'm aware, it was the first USMC manual produced during the Vietnam era, and dated 5 August of 1969 (inner page). There are repros on eBay and originals pop up from time to time. It was also mentioned earlier in the post that raparts sells the manual on CD for $20. The manual is small, about 7" by 5" and a couple of hundred pages.
I know there is some back and forth in the thread about August and December versions, but there may be some confusion as to the publication date of August 1969 and some of the copies' outer covers that show a stamped date of distribution within a unit or section. For example, the picture is the cover of my August 1969 copy showing the stamped distribution dated November of 1969. Other pictures of original manuals have a December distribution date which may have started the discussion.