The first Model AR15 (Model 601) had what is often called a "Duck Bill" Flash Suppressor. It was the only real Flash Suppressor available in Vietnam from the introduction of the Colt model 601 in 1960, until about 1962/63 when replaced with the reinforced "3 Prong" with which we are all familiar.
The Duck Bill is appropriate for models: 601, early model 602 and 607 variants.
3 Prong was much more widely deployed (1963-1967) and can basically be used on any variant in the Vietnam War since so many were already in country by 1968.
"Banned States" 3 prong has fake slots on the side (cosmetic only) and will pass the "Pinky Test" in states where this is necessary.
Made in the USA. Steel. Both are authentic reproductions of the original.
For washer selection (Here). For these we recommend using the lock washer.