Washington experienced another shredding party, this one in full public view, when the Iran-contra hearings resumed yesterday. When it was over, Oliver L. North's credibility was tattered, perhaps irreparably, and the congressional committees faced a new problem involving the Marine lieutenant colonel who stands at the center of the controversy.

They had been given fresh reason to wonder, as a key member acknowledged, how believable North would be if he finally takes the Iran-contra witness stand after all the clamor for his public appearance. And they also had been given a vivid glimpse into North's bold method of operating.

What unfolded on Capitol Hill yesterday was damning testimony about a cover-up attempt involving backdated "phony bills" sent North, backdated "phony letters" he sent in return and elaborate attempts -- including the damaging of typewriter letters -- designed to provide "an action calculated to deceive," in the characterization of Rep. Ed Jenkins (D-Ga.).

It was a real-life story straight out of the spy novels -- not with derring-do romantic James Bond characters, but rather the self-effacing, bureaucratic "Smiley's People" types from John le Carre.

In this case, the operative was Glenn A. Robinette, who served 20 years in the Central Intelligence Agency's "technical services," which, the committees learned, "establish false documents." He now works as a "private security consultant."

Robinette, a retiring, mild-mannered, soft-spoken, white-haired witness, told how he installed an elaborate $13,900 security system at North's house in Great Falls last year at the request of retired Air Force major general Richard V. Secord, a key figure in shipping U.S. arms to Iran and resupplying Nicaraguan contra forces with proceeds from those deals.

Secord wanted the system installed, Robinette testified, because North had received threatening calls, his family had been subjected to harassment at their house and the former National Security Council staff aide had been dissatisfied with the posting of two guards on his property.

Work on the system -- which included remote control gates, alarms, lights, and heat- and smoke-detectors -- began in May last year and was completed early in July. Robinette testified, under persistent questioning, that it was paid for entirely by Secord and that he never expected to bill or receive payment from North.

That fact became critical in the testimony yesterday because it is a federal crime for a U.S. employe "to receive any form of compensation for his services other than his salary."

Last December, nearly six months after the system was installed and after North was fired from his White House job for his role in the diversion of funds to the contras, North phoned him, Robinette said. "He said that, 'By the way, you never sent me a bill,' or 'you've never billed me for that security installation at my residence,' and 'How about -- you better send me a bill,' or 'Please send me a bill.' "

Robinette obliged. He sent two. Both were false, he testified.

The first was backdated to July 2, 1986.

"You prepared this bill in December, didn't you?" Senate committee deputy chief counsel Paul Barbadaro asked Robinette.

"Yes, sir," he replied.

"And that date is false, isn't it?"

"Yes, sir."

"The bill is for $8,000 for the security system. That's not what the system cost, is it?"

"No, sir."

"And that amount is false, as well as the date is false, isn't it?"

"Yes, sir."

The same was true of a second bill he backdated to Sept. 22, 1986, and then sent North. Back came two letters from North, each drafted as if to respond to Robinette's backdated bills.

Robinette acknowleged that what happened was "you had sent phony bills and he sent back phony letters."

North's letters, as Sen. William S. Cohen (R-Maine) pointed out, represented "a dramatic difference in tone and substance."

The first was formal, beginning "Dear Mr. Robinette," and ending "Sincerely, Oliver L. North." The next, sent to "Dear Glenn" with "warm regards, Ollie," suggested a friendly relationship -- and falsely implied, as Robinette testified, that the two had discussed and agreed upon "two payment options" for North.

In it, North apologized for the poor type in the letter, which was filled with broken letter "E's," scrawling a P.S.: "Please forgive the type -- I literally dropped the ball."

This, too, was false, according to yesterday's testimony.

North's letters were studied by "a well-known document examiner," who told the committees it was his opinion that the damaged letters were "made by filing away portions of the type face." He also concluded that "it is extremely unlikely that the damage to the letters could have been made by dropping the typing ball, as Col. North's handwritten note suggests."

It was that kind of testimony about North yesterday that led Sen. Paul S. Trible (R-Va.) to conclude in remarks to Robinette:

"The evidence here establishes that a government official received a substantial gratuity to which he was not entitled. The gratuity was paid, at least in part, from funds generated by the sale of arms to Iran. That you and Col. North and Gen. Secord endeavored to mislead and to cover your tracks . . . .

"What we see here, it seems, is a confusion of the public interests and the private interest. And all this demonstrates once again the corrosive and corrupting effect of generating operations without checks and balances."

CAPTION:Marine Lt. Col. Oliver L. North was described as taking what a lawmaker called "action designed to deceive."