"Why is that man climbing the Concorde out there and pasting that great big letter N on the side of the airplane?"

"To cover up the letter G, of course. Not to do so would be un-American."

Just such a conversation could take place some time in January at Dulles International Airport when British Airways and Braniff International inaugurate their one-stop, same-plane Concorde service between London and Dallas/Fort Worth. Air France and Braniff are going to do the same thing between Paris and Dallas.

The "interchange agreement" will give British Airways and Air France more flying time for the expensive Concordes, which now sit unused overnight at Dulles. For its part, Braniff will be able to offer Texas oil executives and others Concorde service linking Dallas to Bahrain in the Persian Gulf via Washington and London.

Meanwhile, the legal nuts and bolts are something else.

Federal laws require U.S. airplanes to have registration numbers that begin with an N. BRITISH LAWS REQUIRE THAT BRITISH AIRWAYS PLANES HAVE REGISTRATION NUMBERS THAT BEGIN WITH A G. THUS, THE N TO COVER THE G. BECAUSE THE CONCORDE MUST BE A U.S. registered airplane if Braniff is to fly it between Washington and Dallas.

The French laws are so confusing that perhaps as many as two or three French digits or letters will have to be covered or altered before Braniff can fly an Air Franc Concorde. "We're still working on it," said Jerry Cox of Braniff.

This small example of bureaucracy's triumph over common sense has more chapters.

A U.S. registered airplane must be owned by a corporation "lawfully organized and doing business" in the United States. It cannot be registered "under the laws of any foreign country."

British Airways and Air France have organized subsidiary U.S. corporations to own the Concorde when Braniff flies it. That solves that problem. But it also means that the airplane's ownership must change every time a British or French crew gets off the plane and an American crew gets on.

For the registration to be changed, under U.S. law, "It must be shown that the foreign registration has ended or is invalid." Thus, a designee of Britain or France or the United States must be present at Dulles every time the Concorde comes through to certify that the "foreign" registration has been canceled.

Only after that has been achieved can someone climb on the plane and paste on or remove the big N. The passengers never have to leave the plane, which means they'll miss all this.

"I think we have an opportunity here to inaugurate something really special," said a British source who asked to remain unidentified."We could call it "the ceremony of the patch.' At least once a week a brass band could come out and play while the fellow climbs on the side of the plane and places the patch over the G. It could become international tradition."

Air France/Braniff are planning two round trips a week between Paris and Dallas. British Airways/Braniff are planning three round trips a week. The Concorde will fly supersonically only over the Atlantic. It will proceed at a mundane, subsonic pace across the United States. Final approvals are pending, but everybody expects the service to begin sometime in January.