IN AN UNUSUAL gesture to open government, the State Department's Bureau of Politico-Military Affairs recently released a brief memorandum, unheaded and unsigned, on the nation's super-secret nuclear stockpile.

The memo went almost unnoticed by the press, but it was the first time since the bombing of Hiroshima that the U. S. government has issued a public document giving any details of the stockpile.

The apparent purpose of the memo was to make the Reagan administration appear less warlike than its reputation by comparing today's nuclear stockpile with the past. But another purpose of the memo may have been to detract attention from the findings of independent researchers who are beginning to uncover some useful data about the nuclear stockpile, and who disagree with the administration's view of itself as a relative nuclear peacenik.

To date, administrations from Truman's onward have managed to keep the nuclear stockpile Largely a secret. Congress has often made decisions on the military budget without paying much attention to the real contents of the nuclear locker, a state of affairs that has allowed successive administrations to add to and alter the composition of the stockpile without any public notice.

Sen. Brien McMahon, the first chairman of the congressional Joint Committee on Atomic Energy, said in 1949 that the secrecy about the stockpile had made Congress "like a general who must train his troops without knowing how many rounds of ammunition will be issued." McMahon went on, "I fear we lack perspective to pass (judgment) upon any major defense issue."

Though today's lawmakers know more about the American arsenal than McMahon did then, the problem is still real.

The new stat department memo boldly tells us some remarkable facts about the past, but almost nothing about today's stockpile. By giving us figures for the 1940s, the memo shows that when Truman was pushing his adventurous doctrine and the Marshall Plan on postwar Europe he was engaging in an enormous atomic bluff: The American nuclear cupboard was virtually bare.

In 1946, for example, there were components (not the same as assembled weapons) for only nine atomic bombs, in 1947 only 13 and in 1948, the year before the Soviet Union tested its first atomic weapon, 50.

Precise numbers in the memo stop there. It says the stockpile increased through the 1950s and reached its highest level in the mid 1960s. Since then it has, "despite a few fluctuations," declined. Although it is true, the memo says, that the administration is building "several thousands of new (nuclear) weapons . . . almost as many old ones will be retired and disassembled. The net result will be a moderate increase in the size of the stockpile by the late 1980s but the total will still be lower than the peak in the early 1960s."

However, the memo gives no actual figures for the present stockpile, nor does it give us details about the types of weapons now in the arsenal, nor indeed about those that are planned. The memo suggests that the government would be delighted if Congress continues to make decisions on massive increases in defense spending, on the production of new weapons systems such as binary nerve gas shells and on improvements in conventional weapons without considering the types and numbers of weapons in the nuclear stockpile.

Conceivably, if detailed information on the stockpile were part of the ongoing debate on defense issues, public and congressional attitudes might be very different.

For an assessment of today's stockpile we have to turn to last month's newsletter from the Arms Control Association, a nonpartisan, Washington-based group. The newsletter contains the first independent attempt, using public sources, to estimate the size of the stockpile and to examine, in detail, the current upsurge in the production of new nuclear warheads. The article by William Arkin of the Institute for Policy Studies and Tom Cochran and Milton Hoenig of the Natural Resource Defense Council puts the current stockpile at 26,000 warheads (half strategic and half tactical) which is about the same as the total in 1962.

Before leaving office, President Carter secretly authorized the doubling of warhead production. Whereas, during the late 1970s, about 1,000 warheads were produced annually and 1,300 old ones were retired, the 1981 stockpile memorandum, signed by Carter in October 1980 called for a "dramatic increase" in warhead production. President Reagan signed another stockpile memorandum in early 1982 raising Carter's figure, reportedly, by another 380 over a five-year period.

The Arkin article estimates that a massive 23,000 new nuclear warheads would be built during the next 10 years if all of Caspar Weinberger's visions for new weapons systems become reality. An additional 14,000 are identified in current research and development programs throughout the mid-1990s. With concurrent retirement of old weapons, the Arkin article suggests that by the 1990s the U.S. stockpile will total 32,000 -- equal to the all-time high of 1967. An important difference this time, as the article points out, will be the type of weapons.

It would not be simply a matter, as it has been in the past, of taking fissile material, uranium 238 or plutonium, out of the old weapons and putting it into the new weapons. Technological refinements have led to the production of smaller weapons and of neutron weapons, but the supply of plutonium and especially of a third bomb fuel, tritium, from the old weapons would be insufficient to build all the weapons now envisaged.

The amount of plutonium currently produced by U.S. government reactors would need to be more than doubled if Reagan goes ahead with plans to produce the new generation of weapons such as the Trident II, the MX and the cruise missile. Other sources of fissile material will have to be found.

The administration did not like the Arkin article. The Department of Energy, the agency charged with the production of fissile material for America's nuclear arsenal, called in the authors and asked for an account of their sources. The department apparently doubted at first that the information could have come from open sources, and said the authors had "gone too far" in their report. Without confirming anything the authors had written, the officials made it quite clear they disapproved of even estimates of the stockpile being made public.

When many Americans are asking for a freeze on the nuclear arms race, seeking meaningful arms control talks and an open debate about nuclear weapons, some people in the administration are evidently reluctant to oblige. They prefer to follow Truman's example and continue the atomic bluff.