A year before the Iran-contra scandal rocked the Reagan administration, the U.S. Customs Service developed information on one of the early secret arms shipments to Iran by the White House. But Customs did not pursue the matter after it was assured by the Central Intelligence Agency that the United States was not involved.

The near-exposure of the secret Iran arms operation in the fall of 1985 was touched off by an occurrence as trivial as the taped door latch of Watergate: The plane that carried the weapons to Iran flew over Turkish territory on its way back to Israel, and peeved Turkish officials publicly identified the DC8 and its secret route. But the press never picked up on the Turks' disclosure.

This bizarre might-have-been element in the Iran-contra scandal was gleaned from two confidential Customs documents we have seen.

The first document was written on Sept. 20, 1985, by the chief of the Customs Service's office of intelligence, George D. Heavey. It reported that two days earlier, "the Office of Intelligence received fragmentary information . . . that a U.S.-registered DC8 aircraft {with a crew of three} had landed for emergency repairs in Israel directly from Iran and was en route to an unknown destination."

The internal memo went on to say that even though Customs had the tail number of the DC8, the Israeli defense ministry maintained a "blackout on information on the aircraft." As a result, the memo added, the U.S. ambassador to Bahrain could say little when the Iraqi ambassador there "made discreet inquiries about {this and other} flights and arms smuggling to Iran." Customs had no way of knowing it, but its puzzling intelligence information involved the U.S.-approved Israeli shipment of 408 TOW antitank missiles to Iran on Sept. 14, 1985, as part of an arms-for-hostages deal.

Customs intelligence analysts were able to provide more details on the mystery flight in a memo to Commissioner William Von Raab written on Oct. 30, 1985. The U.S.-registered DC8 had "departed Tabriz, Iran, with a crew of three" on Sept. 16, the memo noted, adding, "The plane disappeared over Turkish air space, then arrived at Tel Aviv Ben Gurion Airport for an 'emergency' landing (due to an 'equipment malfunction.') The plane remained in Tel Aviv for about 48 hours."

The memo said the incident first broke when "Turkish authorities broadcast information concerning the DC8, its origin and landing in Israel." The memo added that the information "probably was made public by the Turkish government as officials were displeased by the failure of the DC8's owners to pay the overflight fees."

Finally, the memo stated, "a check with CIA officials on U.S. involvement was negative." Customs' dangerously suspicious intelligence analysts were lied to.

In fact, Von Raab was never let in on the secret operation his intelligence people had almost figured out. On July 11, 1986, Dale van Atta told Von Raab that the White House was smuggling arms to Iran. Von Raab replied that Van Atta's information was wrong.