Preservationists fought hard to save the Old Post Office. They thought they had succeeded when the Romanesque building on the Federal Triangle was designated an historic landmark in 1973.

But now it's gone. Vanished. Pfffft.

A thief snatched the Old Post Office and made away with it sometime late last week.

Not the old stone structure, that is, weighing several thousand tons, with a 350-foot tower.

The Old Post Office snatched, in fact, is about the size of a doll house. Not a mere approximation but a carefully crafted duplicate, correct to the smallest detail valued at $5,000 to $10,000.

It disappeared from the Pension Building sometime between last Thursday night and Friday afternoon.

The FBI is investigating.

The theft was discovered last Friday afternoon by Cynthia R. Field, president of the Committee for a National Museum of the Building Arts. The committee had borrowed the General Services Administration's three Federal Triangle site-plan finalist models for an exhibit, promoting the museum idea, which opens tomorrow.

Field was escorting committee members through the exhibit when she "gasped in horror."

"The thief chose by far the most attractive piece around, because it was elaborately detailed," Field said. "Here we have these three models of the area and the one that concentrates most on the beauty of the building is the Serts, Jackson (and Associates) model because it was detailed in color."

The winning model, by Harry Weese and Associates, is protected by a plexiglass cover.

The missing model was carefully removed, leaving intact mock-ups of the great front steps and surrounding landscapes.

The only investigative lead yesterday in the mystery was a ground-level window found open at the Pension Building near the exhibition area. It has since been nailed shut.

The GSA greeted news of the Old Post Office disappearance with a bureaucratic sigh.

"It's too bad," said a GSA staffer, "because we were going to put the models on display in other federal buildings on a regular basis. But we have two other models so we feel the Old Post Office is still intact."

Huson Jackson, of Sert, Jackson and Associates in Cambridge, Mass., however, thinks the GSA should commission a new Old Post Office model.

"It would certainly be the thing to do," said Jackson after learning of the disappearance.

The museum committee, meanwhile, is pushing ahead with its exhibit, though some of the old fire is gone.

"I can't help but hope that someone would think that the model should be returned and will do it quietyly," said Field "We're really rather devastated by this. All I can say for positive is that at least people will be able to see what the Federal Triangle would look like if the Old Post Office hadn't been saved."

The models, including the one without the Post Office, will be on view weekdays, 10-4 p.m., through Aug. 1.