The intrusion of the Korean jet into Soviet airspace was handled as a routine military matter, Western specialists here now believe. The Boeing 747 was apparently allowed to fly on until it reached an invisible line on Soviet maps beyond which no foreign aircraft is allowed to pass - a line that protects the huge naval and military base at Vladivostok.
Still a mystery is why Soviet pilots and ground radar could not distinguish between the unique shape of the 747 and military spy planes, though sources here say the entire Far East command has been on extra alert for many months for foreign planes that might be armed with cruise missiles.
When the 747 first appeared on early-warning radar screens in the Sakhalin-Kamchatka region, routine messages would have been sent to the operations directorate of the general staff at the Defense Ministry in Moscow.
The messages would not only have provided information but also asked if there were any reasons why the intruder should not be dealt with according to strict rules of interception and engagement already laid down.
''Clearly the return message said 'no - go ahead, it's one plane and not an all-out invasion,' '' says Prof. John Erickson, director of defense studies at Edinburgh University and one of the Western world's foremost writers and analysts of the Soviet military.
''That ruled out firing a missile at the plane from the ground - something the local commanders could have done at any time.''
What intrigues experts such as Professor Erickson is why the Korean plane was allowed to fly for so long over sensitive Soviet territory. Their conclusion: Local ground commanders hoped the plane would veer off and simply fly away. When it did not, it must have come unacceptably close to the line where all foreign aircraft must stop.
''Look at where the plane was brought down,'' Professor Erickson said in an interview. ''It was on a direct route for Vladivostok.'
But why didn't the Soviet pilots realize the jet was civilian and unarmed?
''Maybe they were just unfamiliar with identifying such a plane. All kinds of aircraft go up and down that area. . . ,'' says defense analyst Phil Williams of Southhampton University, who is also a research associate with the Royal Institute of International Affairs at Chatham House in London.
''The Sukhoi fighter that actually fired the missiles at the 747 could not have talked directly to the airliner because it uses different frequencies. Nor was it any use for the Soviet pilots to use IFF interrogation, because the 747 did not carry the equipment to respond.'' (IFF stands for the military procedure of interrogating another aircraft to ask whether it is ''friend or foe.'')
Other background offered by experts:
After a previous Korean aircraft intrusion in 1978, Moscow changed its command structure. Instead of 10 air defense districts, it created five theater commands, of which the Far East is one. Each is supposed to have more flexibility to handle events in its area.
Each has been more than usually vigilant and sensitive in the last 18 months. Professor Erickson, who was in Moscow talking to senior military officers last year, says there has been a definite change in Soviet strategic thinking in that period.
Because of the development of air-carried as well as ground-launched cruise missiles, the general staff now warns of possible attacks from any quarter, not just from the West.
In fact, some senior officers have raised with Westerners in recent months the problem of distinguishing between unarmed large aircraft and those armed with cruise.