Despite three days of inconclusive testimony in a court here about the validity of the so-called Morman will of Howard Hughes, investigators said privately today they can now tell for certain the will is a fake.

The will is one of 35 that have been received at the Clark County Courthouse here purporting to have been drawn up by the billionaire before he died last April 5. The authenticity of all but the Morman will has been discounted by officials.

For the last three days attorneys here have been closely questioning gas station operator Melvin Dummar, named a beneficiary in the Morman will, about his role in its discovery. Attorneys trying to break Dummar's story that he was given the will be a mysterious stranger at his Willard, Utah, gas station now believe the answer lies in a postage meter stamp on the back of the envelope that contained the will.

Although it has not been brought up in court, the Pitney-Bowes Co., which manufactured the postage meter, notified Nevada Attorney General Robert List that it can definitely tell if think in the stamp was used before or after 1969. The Mormon will is dated 1968.

In addition, Pitney-Bowes said in a three-page memo to List's office Jan. 17 that there was only one postage meter at the Desert Inn where Hughes lived in seclusion during the time the alleged will was drawn up. The company supplied the six-digit identification number of that machine to List and others involved in the case, a Pitney-Bowes spokesman said today.

Investigators here said that while the numbers of the postage meter stamp on the envelope are smeared, their sequence conflicts with the Desert Inn meter.

Pitney-Bowes declined today to say whether the postage meter stamp or the will is a fraud. But knowledgable experts connected with the company's investigation said the evidence casts strong doubt on the will's validity and that an examination of the ink would be conclusive.

The memo to List by the Stamford, Conn., postage meter company, also raised another question about the stamp round on the envelope.

"The location of the image on the envelope shows that the envelope could not have been inserted in a normal manner into any Model R. series machine and meter combination," the memo said. A company spokesman said the meter in the Desert Inn in 1968 was the Model R series.

Pitney-Bowes has not had access to the envelope containing the stamp. The spokesman said company experts worked from photographs of the envelope that showed the stamp on the back across the envelope's flap. The spokesman said Pitney-Bowes would not offer its own opinion on whether the stamp and its date were genuine.

The will and envelope are being examined by the FBI at List's request. The Nevada attorney general is considering criminal indictments in the Norman will case.

Dummar, 32, has insisted during questioning here that he did not write the will. He has acknowledged that he delivered the document to Mormon Church headquarters in Salt Lake City where it was found and that he opened the envelope and read the will.

In careful questioning here Wednesday, Paul Freese, attorney for three of Hughes' relatives claiming a portion of Hughes' $2 billion fortune, led Dummar through the series of events after he discovered the will in his service station.

Freese charged in court today that Dummar learned all he could about Hughes to help his wife write the three-page Mormon will. Freese alleged that Dummar researched the material on Hughes while attending Weber State University in Utah and reading the book, "Hoax," written by convicted Hughes' autobiography forger Clifford Irving, and then gave enough information to his wife, Bonnie, so that she could write the document.

Dummar denied the accusation.