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It’s a head-scratcher all right. The signs say “No Standing,” but search the official city directory and its state and federal counterparts and try to find what AWM stands for.

No Standing signs, mostly in Lower Manhattan, prohibit standing at a few locations on weekdays except for authorized vehicles. The only clue to exactly who is authorized are the letters AWM.

Turns out, AWM doesn’t stand for anything at all. The letters were a figment of the imagination of Samuel I. Schwartz, a former traffic commissioner, now a private traffic engineer known as Gridlock Sam.

In the 1980s, he was asked to reserve spaces for certain secretive federal agencies, which did not want to advertise their presence or have their unmarked vehicles identified.

The request was forwarded to the city after a particularly embarrassing intergovernmental episode: F.B.I. agents went inside a building to arrest someone. When they came out with the suspect, their car had been towed.

So the city installed dozens of signs, mostly downtown near the offices of the F.B.I. and the Secret Service, but also in Midtown for agents of the Postal Police and the Drug Enforcement Administration.

“The people who you shouldn’t know who they are,” Mr. Schwartz said.

“I came up with those letters because I thought it meant nothing and nobody would figure it out and it was in the days before you could Google anything,” he said. (Doing an Internet search for the letters today doesn’t help much. The first result in a Google search is for the Association for Women in Mathematics.)

So out of thin air, Mr. Schwartz made up the AWM designation, which the agencies and, presumably, traffic enforcement agents, all wink at. Robert Cassar, the president of Local 1182 of the Communications Workers of America, which represents traffic enforcement agents, said he doubted most people would park in those spots simply because they are mostly adjacent to government buildings.

“I would bet if you look at the summonses we issue you would find very few for people parking in those spots,” Mr. Cassar said.

Mr. Schwartz said he was unaware of any other fictional agency initials. A number of people have tried to figure out what the letters stand for, he said, but nobody has come even close.

“I just randomly picked the letters and issued AWM permits,” he recalled.