BURIED in the State Department archives is the draft of a letter which, had it been sent and acted upon at the time - August 1973 - might have thrown light upon a profound and unresolved international moral issue. That undelivered letter was written by a key career foreign service officer, Thomas R. Pickering, who today serves as United States ambassador to Russia. It is clear what prompted the draft. Maj von Dardel, the aged mother of the heroic rescuer of Budapest Jewry, Raoul Wallenberg, wrote on May 4, 1973, to Secretary of State Henry A. Kissinger asking him ''to undertake something which can throw light on my son's fate.'' Raoul was her firstborn (in August 1912) and, perhaps not surprisingly, her favorite. Maj was only 19 at the time of his birth and already undergoing deep distress from the sudden death of her young husband. (She subsequently married a von Dardel, who became Raoul's stepfather.) Central to Maj's request to Mr. Kissinger was a fact scarcely known at the time. She reminded him that her son's work in Budapest was at the exclusive behest of the US War and Refugee Board. The agency had been created by President Franklin Roosevelt in January 1944. Its office in Stockholm, headed by a certain Iver Olson, arranged for the Swedish legation in Budapest to operate a Jewish rescue mission. Raoul was selected for that task by Olson and the US minister to Sweden. Indeed, the entire diplomatic initiative was a US creation to which Secretary of State Cordell Hull gave formal approval. At the time Maj's letter was received in the State Department, Mr. Pickering served as administrative director of the secretary of state's office. He was obviously fully briefed on the US role, which he characterized in a confidential memorandum to Kissinger as ''the driving force behind Wallenberg's mission to Hungary.'' In the same memorandum, dated Aug. 21, 1973, Pickering proposed that the US government ''ought to take a positive stand'' on the mother's request and ''offer to make new inquiries at the Soviet Foreign Ministry ....'' The recommended action, if implemented, could have had important consequences. Along with the memorandum, the career foreign service officer prepared a formal reply to Maj von Dardel. The draft State Department letter specified that the US would ask Moscow ''what has happened to your son.'' The reply was never forwarded. Across the top of Pickering's memo was the following: ''Disapproved by Kissinger.'' The date: ''15 October 1972.'' Precisely what the fate of Wallenberg was has never been ascertained. Why he was kidnapped by the Soviet military forces in Budapest on Jan. 17, 1945 - over 50 years ago - under a direct order of Soviet Deputy Defense Minister Nikolai Bulganin is not known. Nor is it known what happened to him after his incarceration in the notorious Lubianka and Lefortovo prisons. Moscow, from the beginning, has spun a web of lies and distortion. At first, Soviet officials claimed that Wallenberg was being protected by them. Then they denied (for a decade) that he had ever been on Soviet soil. Finally, they sought to demonstrate that he died of a heart attack in July 1947 and was then cremated. Observers found mystifying the claim of a heart attack. Wallenberg was only 35 years of age at the time and known to have been in excellent health while in Budapest. Especially puzzling was the reference to cremation. Research a few years ago showed that Wallenberg's name did not appear in the carefully kept list of all cremations in 1947 in Moscow's only crematorium. The actual facts about the Holocaust hero remain locked up in KGB files. Only a couple of years ago, Russia's top archival official, Rudolf Pikhoya, bitterly complained in a Moscow publication that the KGB had deliberately classified various documents on the Wallenberg case ''as operational intelligence'' and, thereby, barred them from public scrutiny. Several months earlier, a researcher from the liberal Memorial Society, Vadim Birstein, was suddenly deprived of access to the Central State Archive where he had been examining materials related to Wallenberg. Mr. Birstein disclosed that about 20 KGB officers who had been involved with the Wallenberg case were still alive (in 1991), though retired. However, journalists and researchers could not interview them without permission from the KGB. Why the Pickering proposal was denied is puzzling. But, then again, the US government at its highest levels has never become directly involved. In recent years, American diplomatic officials, like Ambassador Max Kampelman and Ambassador Warren Zimmerman, have raised the issue at Helsinki forums. And a key State Department human rights aide, Ambassador Richard Schifter, unsuccessfully sought information from Soviet Foreign Ministry officers. Clearly, however, it has not been a high priority of any administration since 1945. What makes the relative official indifference dismaying is the fact that Raoul Wallenberg was employed by a US government humanitarian agency - the War Refugee Board. Moreover, he was given the almost unique distinction of being named by Congress (in a bill signed by President Reagan) in 1981 as ''Honorary Citizen'' of the US. Only Winston Churchill, among foreigners, has ever been extended that special status. On Memorial Day, President Clinton pledged that he would ''leave no stone unturned'' to obtain full information on Americans still ''missing in action.'' The pledge could appropriately apply to America's ''Honorary Citizen.'' It could begin with having the author of the unsent memorandum, Ambassador Pickering, now act upon his proposed recommendation. The next stage should involve intervention by high levels of the US government.