The "A1" birdcage actually was used as early as 1965 (4 years after the AR15 was introduced into Vietnam with the advisors and later SEALs). The flat prongs not only caught on the ubiquitous "wait-a-minute" vines, but would also catch rain and direct it into the barrel if the barrel was held with the barrel angled up. Remember that it rains pretty solidly for 4-5 months straight in Vietnam...
Still, it took some time before older ARs were cycled out of use and send to armorers to have components upgraded, so the older 3-prongs were still occasionally seen on rifles much later in the war.
-Troy