I STARE at the picture and stare at the picture, yearning to make the middle-aged man with the moustache metamorphose into the breezy, fresh-faced California kid I knew half a lifetime ago. And fearing the feelings if he does.The grainy photo of the three Americans said to be POWs in Vietnam is a hope and a heartbreak to the families and friends of those men who remain unaccounted for after all these years. The one on the right has been identified by people close to him as Larry Stevens. It's hard for me to say, although I knew him well. Is it because of the change in appearance and poor quality of the picture? Or because knowing that he is alive - after "knowing" for so long that he was dead - would be too hard to take? We were squadronmates aboard the aircraft carrier USS Coral Sea when he went down on a night mission over Laos in 1969. His flight leader, Lt. Comdr. Jim Meehan, suffered aircraft damage as well but made it back out over the Gulf of Tonkin, where he ejected and was picked up by the rescue helicopter. The next morning, Lt. John Paron and I flew low and fast in our A-4 Skyhawks over the area where Larry had disappeared, calling for our fellow "Diamondback" (the squadron call sign) on the emergency frequenc y. There was no answer. As a veteran of Vietnam air combat and Pentagon reporter during the Reagan years, I have followed the POW story closely. I have talked with former POWs, including Sen. John McCain (R) of Arizona, who had been one of my flight instructors, and retired US Navy Capt. Eugene (Red) McDaniel, who obtained the recently released photo. Yet I am not one of those who believe that large numbers of Americans are being held by or with the knowledge of the Vietnamese government. Nor do I believe that US government officials have been trying to bury or cover up the truth. As governor of California from 1967 to 1975, Ronald Reagan supported POWs and their families - because he was touched by the issue and also because so many were constituents. It would have made no sense for Reagan as president to do anything but get at the truth about the 2,000 or so Americans unaccounted for in Vietnam and Laos. Over the years there have been breathless reports of POW "sightings," some by soldiers of fortune like the shadowy James (Bo) Gritz, some by conservative organizations trying to raise money. But no hard evidence. Mr. McDaniel's group, the American Defense Institute, has in the past been accused of "concocting" sighting reports. This is not to say there are no American servicemen alive in Southeast Asia nearly 20 years after the United States stopped fighting there. Some, like Robert Garwood, the ex-marine who reappeared years later, stayed voluntarily. A few may have gone insane in isolated captivity (not surprising, given the circumstances) and been felt by their captors to be unfit for return. It is quite conceivable that Soviet or Chinese intelligence agencies obtained American pilots for their knowledge of advanced US aircr aft. OTHER airmen, like Larry Stevens, who was shot down far south of Hanoi, may have been held for years by disgruntled villagers who were understandably mad at having bombs dropped on them night after night and refused to turn their prisoners - now forced laborers - over to the North Vietnamese Army for shipment north. In such situations, an Army unit on the move may be just as happy not to have prisoners to attend to. After the war, Vietnam was an economic basket case, and soon was involved in another war in Cambodia. The Hanoi government was unable to account for many thousands of missing Vietnamese soldiers, let alone enemy combatants. The Vietnam POW issue has gnawed at America's political gut for years. Part of it is guilt at ignoring for so long the veterans of that sorry episode, the failure to "separate the warrior from the war." In fact, the ratio of servicemen unaccounted for to those killed is quite low in Vietnam compared with other recent wars. In Korea, more than 8,000 remained missing, which is 15 percent of those killed in action. In World War II it was 22 percent (78,000 missing). In Vietnam it was less than 5 percent. But these are numbers on a chart, lines on a graph, not individual human beings whose names are chiseled into the black wall of the Vietnam Memorial - but perhaps ought not to be. And so we stare at the picture and stare at the picture.