Over the years, Trijicon has been making incremental changes in the ACOG line of scopes. Here are a few pictures that show a M150 model made in 2013 and a couple year old TA31RCO-M4 model to show some of the newest little changes.
One of the more noticeable external changes is that many of the newer 3.5x and 4x scopes have the bosses that allow for mini-red dot mounting.
The top ACOG has the bosses (ears near the eye piece) to allow for piggyback mini-red dot mounting. The only downside to having the MRD bosses is that you can no longer slip on a flip up lens cap on the eye piece.
A smaller less noticable external change is that newer ACOG’s no longer have a bible verse on the side of them. This was removed after it was made widely known in the public, and we wouldn’t want a bible verse on our military rifles because we don’t want to offend the people we are killing.
Formally the ACOGs had JN8:12 marked on the side after the model type.
Many new ACOG scopes come with 1/2 MOA turret adjustments now instead of the old 1/3 MOA coin slots. While this is a coarser adjustment, the new turrets are much easier to adjust, and increase the water resistances of the optic. I like how some of the newer ACOGs are marked with clear external markings showing which way you need to adjust the turret to move the point of impact.
Trijicon has also changed their reticles. Now more of the reticle is illuminated then before on the many of their ACOG scopes. While Trijicon has changed the amount of illumination in the tritium only TA01 series of ACOGs often, this last change increased the the illuminated area on the TA31 red chevron line of scopes. Previously these had the red chevron and the 300m post illuminated, but now the 400m and 500m lines and much of the bar leading to 600m is illuminated. Trijicon’s reticle pictures on their website have been updated to reflect these changes.