Those were some really old M16A1s. The 3-prong FH was dropped from the M16A1 in Dec. 1967. Even the later XM16E1 and Model 604 were eventually outfitted with A1 FHs before ceasing production, but most were produced with 3-prong FHs. The good thing about the 3-prong and A1 birdcage FHs is that they did not need to be timed. The A2 FH does because of the closed portion needing to be on the bottom.