The three-line classifed ad in two San Diego newspapers read as follows: "(2) Rubens for sale, $1,250,000 each. Principals only. Contact: Archer Agency, West Coast Rep."

The Archer Agency, as it happens, is a Washington-based "one-man entrepreneurial thing" run by former U.S. Attorney Robert A. Ackerman, who has been in private law practice here since 1971. "I'm not the principal," said Ackerman, "I'm doing this for a friend."

The friend is Washington economist Jack Ben-Rubin, who said yesterday that he didn't expect this to take off like it did. It was only meant as a trial balloon.

Thus far, the trial balloon has drawn more attention from the press than from potential buyers. According to Ackerman, the National Gallery of Art is the only "principal" to call thus far.

"It was just a routine inquiry," said John Hand, the National Gallery curator who made the inqiry.

Ackerman and Ben-Rubin declined yesterday to give details about the paintings or their whereabouts. A story in the San Diego Union last week quoted Ackerman as saying that a university economist in the Washington area had come to him a fter a Majorca vacation where he had met an art dealer who knew of a private art collector with two Rubens paintings to sell.

Ackerman and Ben-Rubin declined to comment on the Union story. "We need a little more time to firm up the business arrangements," Ackerman said.

But Ackerman did say that his client had been approached by someone who said, "If you're going back to the States, they're rich back there, maybe you can sell the paintings." A commission is customary in such cases.

Most old master paintings in the $1 million price range are sold in the rarefied atmosphere of High-priced art galleries in London or New York, or at auction. "Anyone with a million-dollar Rubens to sell dosn't place a half-inch classified ad," said Henry Gardiner, director of the Fine Arts Gallery of San Diego, in a telephone interview last week.

"You do it appropriately, by sending a brochure to various museums. It's all very odd. An island in the middle of the Mediterranean is not an average place for a Rubens to appear. It just doesn't set right."

But he did add that if these were bona fide paintings by the Antwerp master Peter Paul Rubens (1577-1640), that the price of $1,25 million was not out of line.

"It's all legal, and we're assured of the authenticity," said Ben-Rubin. "We're also clear that no principal in his right mind would buy without authenticating the works."

The ad cost the Archer Agency $35 for seven days, and though it is an unusual way to market $2.5 million worth of old master paintings, it was not unusual enough for the National Gallery to ignore it.

"You never know," said Hand. "Every once in a while something does turn up."