One idea might be to paint a few packages from nearby stores — like Apple, Ralph Lauren or Louis Vuitton — as if they were being glimpsed through the windows of the imaginary facade. These would not be the first nonarchitectural elements in the mural. Mr. Haas included a cat on a sill in the original. Ms. Vogel, who has lived in the building since 1974, said it was her cat.

Image The building's co-op has not announced a decision on restoration of the mural, which has been marred by decay and graffiti. Credit... Michael Appleton for The New York Times

It must be possible to find $250,000 to reclaim a milestone of public art. For the price Liu Yiqian was willing to pay this week for Modigliani’s “Nu Couché,” you could restore Mr. Haas’s mural annually for the next 681 years.

Though the building is in the SoHo-Cast Iron Historic District, the mural is not included in the 1973 landmark designation report, because it did not yet exist. A permit to restore it could be issued by the staff of the New York City Landmarks Preservation Commission without being reviewed by the full body, a spokeswoman said.

The mural was inaugurated 41 years ago by a nonprofit group called City Walls, founded by Doris C. Freedman to bring artwork to blank, windowless lot-line walls. Many murals of the time were abstractions or figurative paintings on social themes.

Mr. Haas took a different approach. He imagined what the building’s east facade would have looked like had it continued around the corner. He produced drawings based on visual observation and what measurements he could obtain, working with the architect Hanford Yang, whose office was in the building.

The artist did not get on a scaffold to do the job himself. City Walls contracted with Van Wagner, which specialized in outdoor signage. Two sign painters finished the job in January 1975 after about two and a half weeks, exciting much comment among Mr. Haas’s peers, coming and going from the nearby Fanelli Cafe.