"Sandakan 8" an unheralded but extraordinarily stirring Japanese movie now at the K-B Fine Arts, was one of the five obscure nominees for best foreign-language film in the 1975 Academy Awards ceremony. What a pity it didn't win! An Oscar might have meant as much to "Sandakan 8" as it meant a year later to "Black and White in Color," a far less impressive and accomplished underdog candidate.

Ironically, the 1975 award went to a movie directed by a Japanese, Akira Kurosawa's "Dersu Uzala," a co-production listed as the official entry of the Soviet Union. For both technical and artistic reasons the Kurosawa picture, a pastoral epic shot in 70mm, seems to be considered a commercial longshot in this country, and dedicated moviegoers may judge for themselves when it is shown this weekend - as part of a Soviet film retrospective - at the American Film Institute Theater.

"Sandakan 8," on the contrary, appears conspicuously accessible. It's a soberly inspirational chronicle of suffering and endurance, evoking elemental, heartfelt emotions without going simpleminded or mawkish. Its subject happens to be the fate of history's forgotten women, exemplified in this instance by a spindly, hunched, solitary old woman called Osaki, who was sold into prostitution at the age of 14.

The director, Kei Kumai, is not familiar to American art-house audiences, so the dramatic strength of the material and his ardent, conscientious sensibility come as a surprise. Now 47, he has been directing since 1964 and appears to be both typed and respected as a sound storyteller and craftsman with a preference for socially-conscious themes. "Sandakan 8," released to considerable acclaim in Japan three years ago, was inspired by a series of articles by a woman journalist, Tomoko Yamazaki, whose equivalent in the film is a prim, sincere, timorous young researcher named Keiko, played by Komaki Kurihara.

The movie appears to combine what may have been the journalist's ambigous and somewhat guilt-ridden relationship to her subject with a broader vision - perhaps the writers's, perhaps the filmmaker's - of the historical costs of feudalism and female servility to Japanese society. Osaki's ordeal carries an intimate dramatic impact, but that impact is enhanced by a historical perspective, which views her and other peasant girls who shared her fate - the Karayuki-san - as pawns in a strategy of colonial expansion and settlement pursued by Imperial Japan.

The title refers to one of the nine Japanese-owned and operated brothels in a port city of northeastern Borneo, a possession of the British Empire when Osaki is transported there in 1913 from her home village. Amakusa on the island of Kyushu. The scenario is constructed along three intersecting time tracks. The movie begins and ends in the present, with Keiko's pilgrimage to Sandakin, now a part of East Malaysia, where she searches for the sites of the brothel and a prostitutes' graveyard located somewhere in the jungle.

This quest triggers recollections of Keiko's first chance meeting with Osaki three years earlier on Amakusa. A chance remark leads Keiko to believe that she has found one of the surviving Karayuki-san she has been for, but her fear of spooking the old woman leads her to conceal her own motives. She befriends Osaki, moves in with her and eventually coaxes her into revealing the story of her life, which plunges the film into a more remote past.

Keiko's affection for Osaki is quite sincere, and the old woman is gratified by it, choosing to identify her inquisitive young visitor as the daughter-in-law who has, in fact, spurned her. However, Keiko's false position preys o nher conscience; she realizes that she's using Osaki too, even if her form of exploitation can't be equated with the form Osaki has already endured. Eventually, she breaks down and tearfully apologizes for not being candid about her motives with the old woman, who gently consoles her. Moments later it comes as a painful confirmation of the differences in status, upbringing, experience, age and personality separating these two women, despite their mutual affection for one another, that Keiko cannot console Osaki when the prospect of the young woman's departure overcomes her.

The movie would be worth seeing for one feature alone, the magnificent performance of the late, illustrious Kinuyo Tanaka as the old Osaki, Tanaka, who died last spring, began her screen-acting career in 1924, at the age of 14, and she became the first Japanese woman to direct a theatrical film.

Tanaka has a facility for projecting contrasting moods of anxiety and contentment that are disarmingly powerful. The quickness and the touching transparency of her expressions seem to quicken and expand your own susceptibilities. She's an arresting and sensitizing presence on screen.

In addition, Kumai enjoys the bonus of a remarkably strong performance by Yoko Takashy, who plays Osaki as a young woman. Actresses portraying the same character at different ages rarely complement each other as effectively as Tanaka and Takashy have in "Sandakan." As a rule, it's difficult to accept the illusion that one presence could have evolved into the other. Here one believes it.

Kumai's methodical dramatic preparation seems to be indispendable to the surging emotional impact of the climactic sequences. Many of these sequences are stunning, especially the depiction of Osaki's sexual initiation at the hands of a lean, savagely tatooed customer, a passage of brilliantly visualized erotic shock and helplessness that makes all the explicit depictions in Oshima's "In the Realm of the Senses" seem even more plodding and academic.

Kumai stages a lyric aftermath which keeps this overwhelming rape scene reverberating for into the night: A nude Osaki staggers into the courtyard in a drenching rainstorm. Far from washing the shock away, this particular visual metaphor has the ironic effect of branding the preceding scene even more deeply. It's not likely that anyone who gets caught up in this story will forget the image of Yoko Takashy with her head thrown back as the key hanging from a necklace worn by the tattooed man rubs across her face.