For 2 1/2 years District of Columbia city administrator Julian R. Dugas apparently has been acting as chairman of the D.C. Alcoholic Beverage Control Board without a formal order of appointment.

When a mayoral aide was asked about the matter two weeks ago, he told a Washington Post reporter that Dugas's reappointment - after an initial term that expired in August 1975 - had simply been "misplaced." The same official, executive secretary Martin K. Schaller, subsaquently produced a document that he identified as an approval form signed by Mayor Walter E. Washington in 1975.

Schaller acknowledged yesterday that both the approval and the actual reappointment order had in fact been executed much more recently, and were backdated.

Schaller made the admission after being shown evidence that the approval form was a photocopy, not an original and had been copied from the reappointment order, which in turn bears a 1976 watermark.

"It's a very simple matter for (appointments) to get misplaced or lost," Schaller said. Not long ago "we were about 700 documents of this kind behind. . . In many instances there were no orders and we simply reconstructed them."

Schaller said his usual practice, when such an order had been lost, was to search for a written record showing that the mayor had approved an appointment at the time it took effect. But aprovals as well as orders, Schaller acknowledged, have sometimes been "reconstructed."

Another city official involved with appointments said the Dugas was a mild one compared to instances in which the city had named dead or nonexistent people to some city governments boards, committees and commissions.

The question of Dugas' membership on the ABC board was raised by attorney Louis N. Nichols, a former board member who represents Guncher's a Georgetown restaurant with a case currently before the board.

Nichols said yesterday that he was considering seeking a court injunction to stop Dugas from casting a vote in the Guncher's case. But Schaller and other city officials have taken the position that the existence of a formal order reappointing Dugas is immaterial. They argue that Dugas can continue to serve so long as no one is appointed to replace him.

It was Nichols, along with his partner, Claire O. Ducker, who emerged the services of "question documents" expert John H. Orr, who discovered the 1976 watermark.

Orr also supported Nichols' charge that two signatures on the approval form - Schaller's and the mayor's - appeared to have been gone over to a second time in heavy ink, giving the impression that the document was an original Schaller said yesterday that he was unable to explain who might have gone over to the signatures, or why.

Dugas, one of the mayor's closest advisers, said yesterday that he knew nothing of any challenge to his ABC board membership. "Nobody's raised that to me," he said.

He added that it was his practice not to attend ABC board hearings unless his presence was required to provide a quorum. Otherwise, he said, he leaves such matters to his two colleagues, James W. Hill and Arthur W. Jackson.

The Guncher's case marks the first time Dugas has actually participated in an ABC board hearing since Jan. 24, 1975, but he has signed a number of orders and opinions since then in his capacity as a board member.

Hill and Jackson, like Dugas, have full-time posts with the city government in addition to their part-time, unpaid service as members of the ABC board, Hill is the director of the city's economic development agency and Jackson is the chief hearings examiner for the Human Rights Commission.

The ABC board reviews all applications for liquor licenses and license renewals in the city, and is responsible for imposing punishment when it decides that a restaurant, private club or liquor store has violated the city's complex ABC regulations.

Dugas said he had agreed to hear the Guncher's case at the request of Hill and Jackson. "They asked me to sit, and I sat," he said. "It's as simple as that . . . I don't belong to the board then I can be disqualified, and that would be fine with me. I have no interest in this case whatsoever."

The case, now awaiting the board's decison resulted from an incident in September 1976 when a group of students from Marymount College, all under 21, attended an "anniversary party" at Guncher's, at which complimentary drinks were served. Several students were treated at their school infirmary, and no one was admitted, unconscious, to the intensive care unit at Arlington Hospital. Her condition was diagnosed as "acute alcoholism," according to Linda McMahon, Marymount's dean of students.

Although a police investigation of the incident was completed approximately a month later, the ABC board did not file charges against Guncher's until August 1977. As a result of that delay Guncheh's attorney asked the board to drop the charges, and the two sitting members, Jackson and Hill, split on the question, Jackson supporting the Guncher's motion and Hill opposing it.

It was then that Dugas entered the case, apparently to resolve the disagreement between his two colleagues.

After voting with Hill to deny the motion to dismiss charges, Dugas attended three separate hearings on the matter, the last of them held Jan. 18.

The hearings were marked by several hostile exchanges between board members Dugas and Hill, on one side, and attorney Nichols, on the other. Nichols is a former member of the ABC board who resigned after being observed drinking after hours at Clyde's 12 years ago. A friend and neighbor of Jackson's Nichols regularly drives Jackson to work.

Nichols said he and his partner Ducker, might go to court in quest of an injunction to prevent Dugas from casting a vote in the Guncher's case.