The radar echo in this case seems to have been coincidental. The Record Card statement that the unknowns were seen visually and on radar is just a mistake and is not supported by the original reports, which clearly say this single echo approached from the opposite side of the plane and passed by having all the characteristics of another aircraft (some u/i aircracft were encountered in the region an hour or two later).
Some of this is hard to read but I can't help but wonder about our old friends the fire balloons. They're a pest in the modern west; they were traditional in many places in that part of the world at all sorts of festivals. I believe there are Japanese fire festivals in March (?).
The gist of *most* of the descriptions seems to be: Dozens of small orange-ish balls, no radar contact, silent, glowing but not brilliant, appearing in groups or "strings" of a few at a time, drifitng aft of the plane (some said they were not moving at all, others that they were moving slowly but always moving in the opposite direction to the plane), the lights in each group lighting up and going out in sequence, front to back, seeming to keep fixed formations. These are all typical characteristics of fire balloon sightings with which we have become all too familiar in the UK in recent summers.
Against that theory is a) the altitude (most observers seemed to think the lights were roughly at the P2V's altitude of >12,000, which I think would be extremely high, but this could be hard to judge) b) none of the objects seemed to stay lit for very long - seconds, not long enough to have climbed any distance (how would they suddently ignite when already at tousands of feet?) and c) of course the main obstacle is: Where the heck would they have come from?
The location is 37deg 25'N 132deg 35'E in the middle of the Sea of Japan. The aircraft was headed 125deg magnetic. This is 130 miles from mainland in any direction, 80-90 miles from a couple of islands, nearest dry land of any kind a srap of uninhabited rock 40 miles away. In the direction of the objects off the port wing (NE) there is nothing for 500 miles as far as I can see. They would have to have been released from a ship. I suppose I can just about imagine susperstitious fishermen or whatever adhering to custom and launching fire balloons for luck on some festival date, but it doesn't really wash, does it?