They were imported with fake, non-threaded thread protectors welded on them. The muzzle is thinner than other AK-74 barrels. They can take a 1/2x28 thread. The military Romanian 5.45 muzzle brake fits a threaded front sight base that is threaded 22 mm. The muzzle brakes were popular retrofits during the AWB because the ATF ruled the Romanian muzzle brake a non-flash hider. The standard AK-74 muzzle brake was ruled as being a flash hider. Many of the Romanian ones that were imported were fitted to AR15's and other 223 rifles by welding. This caused a surplus of threaded Romanian 5.45 front bases and a shortage of muzzle brakes that thread on them.
The Romanian 5.45 barrel is narrower in diameter than an AK-74 from the gas block to the muzzle. The standard AK-74 parts are not a direct fit. 14 mm left hand threads are too large for the small diameter muzzle of the Romanian barrel.
Despite some media reports, there were no AK-47s involved in the incident