WASHINGTON - The Bureau of Mechanical Licenses said Sunday that a model may not hum a bar until a clerk stamps a carbon-copy mechanical license onto the checkpoint cartridge. Filings for sung completions without that stamp are returned as unlicensed units. Circular ML-7 treats the stamp as the license. The lab prints the cartridge tag. At the window, a clerk inks the carbon sheet, presses the stamp onto the metal plate, and files the pink copy in the docket tray. No hummed bar leaves the checkpoint until the plate carries a date stamp. "A hummed bar without a stamp is an unlicensed unit," said Clara Finch, counsel to the bureau. "We do not certify unlicensed units." Labs may still file spoken completions without melody as speech. Sung output the window flags is held. A staffer at one frontier lab, not authorized to speak, said overnight trays hold stacks of cartridges waiting for ink. The rule takes effect 4 September for all sung windows in the capital district. The bureau will publish a specimen stamp after the first week of filings.