1) The first "official" scope set for the AR15/M16 was the Delft scope made in the Netherlands, this scope was a slightly modified version of the one originaly designed for the AR10 & fitted to a mount specifically for the AR15/M16, the mount was marked "Colt-Armalite" & had a Colt a part #, the scope wasn't mkd Colt, they were purchased by Colt from the Netherlands & supplied along with the 601's (& possibly some 602's), the scope & mount are both shown in the original Colt AR15 (601) manual & the original Air Force 601 manual. These are believed to be 63 & earlier.
2) The next "official" scope for the AR15/M16 is the seemingly common Colt marked scope on the "standard" mount, these scopes were made for Colt & marked Colt on them, the line:
1) early (64 or so) 2.75x, mfg under contract to Colt by Realist & marked Colt "made in USA", reportedly the Army bought a number of these w/the inverted post reticule & there's a version w/the standard crosshair reticule which also showed up & were used on rifles in VN, a lot of these scopes were used there but were "private purcahse".
A bit later these were made in 3x or marked as such, it's believed Realist made some 3x's but since they were contract scopes it's hard to "pin them down". There are a few of these scopes marked "Armalite" but not Colt, these scopes were actually bought from Realist by Armalite for the AR18/AR180 between 1965-1970 & are identical to the Colt scopes & will attach directly to a Colt mount, the mounts for the AR18/AR180 are different than the mount for the AR15/M16 but the pattern for the mounting screws are the same, Colt dropped "Armalite" markings from AR15 products in 63 (during 602 production) & these scopes are post 63.
3) In the mid-late 60's Colt switched their scope contract to a Japanese Co, reportedly Hakko, 67-68? & these scopes are marked Colt 3x, Realist had begun developement of ART (auto range telescopes) in an attempt to sell them to the Army (& commercially) at this time & it's not known whether this influenced them dropping the Colt scopes or Colt simply got a better price for their contract scopes to be sold w/AR15's/M16's from the Japanese Co.
4) The next version is the more common Colt 4x, again mfg under contract to Colt by the Japanese, early to mid 70's.
To "sum up", the Delft scope sets & early inverted post Realist scopes are reportedly the ones actually purchased by the Army on a small scale but any of the above scopes could have been used as they were actually a commercial product & available & many were privately purchased & used in VN along with a lot of other commercial scopes, Realists, Redfields etc.
There was ongoing attempt by the Army during the war (officially & "unofficailly" by in theatre armorers) to develope "useable scope mounts/scope combinations for use w/the M16. I saw a number of different combinations myself there in 71-72, all "unofficial" but used non the less.