They used differing methods to disable the threads on preban guns released as post bans. They welded nuts over the threads (get the parts count in line, cut the welds and unscrew the cap). some were knurled over and left on the rifle (I've bee able to screw down a muzzle brake over the knurling and get the threads back) or turning them off completely (can't do much with this other then rethread 1/2-28). Maybe yours were knurled over or had the cap someone removed.
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