It appeared in an American paper the day after the U.S. senate vote on the Panama Canal: an advertisement announcing the sale of water from the Panama Canal. For $4.50, sent to a Panama City post office box, diehards were given the chance "to own a piece of the Panama Canal in perpetuity."

Each bottle contained four ounces of Panama Canal water as a lasting memento, the ad promised. Included in the deal was a "do-it-yourself treaty" texts as approved by the U.S. Senate, with open spaces to fill in your own amendments, reservations, articles.

When it comes to the canal, history has shown, Panamanians are sensitive and so was Guillermo Valdes, an enterprising television journalist who spotted the ad. Valdes set out to unmask the mysterious "Canal Corporation" in whose name the Post office box appeared. He came up with the names of four Americans living at Fort Clayton, a U.S. Army base in the Canal Zone.

"It's a hoax, a scandal, an affront to the nation," Valdes bristled on the air and proceeded to telephone the advertisers. Terry Lylee, one of the four, unwittingly made things worse.

"It was not meant to be a joke, it's a real business," a nervous Lylee told Valdes. The bottle was nicely decorated and the treaty text beautifully designed.

"Americans are really unbelievable," said Valdes who announced that he would not rest until the government ordered the post office box closed down.

WHEN SEN. DENNIS DeCONCINT (D-Ariz.) visited Panama in December he got the standard senatorial tour: talks with opposition leaders, with people in favor of the treaties, with Zonians and student groups. Last week some of DeConcini's Panamanian acquaintances swapped stories about this quiet American whom no one really paid much attention to yet whose intervention clause brought Panama close to dumping the pacts.

None of these Panamanians wanted their names used for fear fo being publicly associated with the "colonialist senator" but all agreed he seemed "a nice guy, somewhat naive, and with little idea of what the canal issue or Panama was all about."

Unlike other senators, who had traveled with colleagues, DeConcini arrived with his own private group, including his wife and his mother. To Panamanians, who, like most Latins, are strong male chauvinists, this was quite a shock.

"How is it possible, he kept checking with his mother, and his wife asked more penetrating questions than the man himself?" asked one indignant opposition lawyer with whom the DeConcini party had a long talk.

One such meeting with opposition leaders took place in the home of a Panamanian physician Osvaldo Velasquez, where the Panamanians reportedly asked the Arizona senator which way he would vote on the canal pacts.

"You know what his answer was?" one of the men present at the gathering recalled. "He said I have three children, two are against and one is in favor and I still haven't made up my mind."

At the meeting, DeConcini showed much interest in the human rights problems for political opponents to Panama's military government and asked if there would be violence in Panama if the treaties did not pass. Afterwards, he sent a letter expressing his "thanks and admiration."

"Both my wife and I . . . have a much better grasp of the implications of the treaties for domestic Panamanian politics," DeConcini wrote.

"Imagine, we thought DeConcini was a nice guy," said one of the opposition leaders, a prominent lawyer in Panama.

At the end, before he left, he even gave us his card and said we could contact him any time. If ever we had any problems, he could kick up a storm, in the Senate for us. Famous last words."

THERE WERE NO demonstrations or angry banners last week in the residential neighborhoods of the Canal Zone. Just here and there, those small, telling signs: "Garage sale."

Quite a few of the American residents say they will wait and see how things go when Panama starts taking over the police duties, the courts and the stores.

The Senate vote approving the treaties was not exactly a shock to pain when a relative dies after a long disease," as one old-timer here said. "You knew it was coming, but to hear it happen was agony."

The main effect among the dyed-in-the-wood patriots who proudly say they have more boy scouts per capita than any U.S. community, appears to be disillusion with what they believed to be "American democracy."

The horse-trading the vote-swapping, the wheeling and dealing on the Senate floor before the vote had nothing to do with what the American people think about the canal, many Zonians claimed. And not a few felt that the canal should have been made by a national referendum.

AT THE BALBOA Yacht Club the young, tanned crowd did not seem to share the gloom of their elders. In front of the wooden frame building on the canal bank, car radios blared as usual. Indoors, under the lazy fans that always appear in novels swapped stories about fishing trips even getting high. The bar itself is even being remodeled as though the Americans are here to stay.

They did not seem to care much about politics. If necessary, although many of them were born in the Zone, they could replace this waterfront with another in California or Florida.