OR SHOULD it be "Dear Ming Toy"? We are only trying to think through the possibilities raised by the Postal Service's assistant general counsel, Charles R. Braun, in a statement the other day. Argument for a reimposition of the Postal Service's authority to monitor people's mail - anyone's mail - for reasons of vaguely stated "national security," lawyer Braun came up with a truly inventive justification. "If there was no national security mail cover program," he explained, "the FBI might be inhibited in finding out if a nation was planning war against us."

Well, now. You probably hadn't thought of that one. The reason you hadn't, of course, could well be that during all those years in which mail covers for "national security" reasons went unchallenged, they didn't seem to alert anybody to the big troubles - to the painful episodes in which we or people and countries of interest to us were deeply involved. There was the Soviet introduction of missiles into Cuba and there was the Yom Kippur War and there was the Soviet march on Prague and there was the Tet offensive and - importantly - during all of this there were also national security mail covers.

We will not press the obvious argument that the intelligence agencies may have been so bemused by things like relying on local mail covers that they missed the meaning of troop mobilizations and comparable stirrings elsewhere. The point is merely that this dangerous, hit-or-miss practice was rightly curtailed by U.S. District Court Judge Lawrence Whipple, who declared that the phrase "national security" was simply too "ambiguous and broad" a justification for such surveillance. If you have any doubts about the wisdom of Judge Whipple's ruling you need only consider the case that triggered it: A schoolgirl in New Jersey had written to the Socialist Worker's Party in 1973 for some information for a high school project. Her action brought an FBI agent to her school.

The Postal Service has responded to adversity by trying to overturn Judge Whipple's ruling - but not in court. It has said, in the Federal Register, that it intends to continue authorizing this kind of cover. It has redefined its mission as protecting the country against "actual or potential threats to its security." That brings us back to "Dear Ivan" - and the rest: "How about starting a war with the United States? It's worth thinking about, don't you agree? Anyhow, let me know your reaction.Best to the little woman. . . "

Undoubtedly (and happily) this effort to reverse Judge Whipple's ruling will be challenged. The only aspect of the Braun fantasy we hate to give up is this: the substantial comfort - even the overwhelming sense of national security - that would flow from believing that enemies of this country planning a war against us were reduced to relying for their communications on the U.S. Postal Service.