B757 AND A320 RECEIVE TCASII RA FROM ACFT PENETRATING BOS CLASS B ERRONEOUSLY.

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ON FINAL APCH TO SEA RWY 16R; ABOUT 4 OR 5 MI OUT (GEAR DOWN; FLAPS 20 DEGS); TWR CTLR ADVISED US OF UNIDENTED TFC AT 12-1 O'CLOCK POS; SLIGHTLY BELOW US. WE VISUALLY ACQUIRED TARGET SHORTLY THEREAFTER. ALMOST SIMULTANEOUSLY; TCASII GAVE US A TA INDICATING THE TFC ABOUT 200-300 FT BELOW US. I DISENGAGED THE AUTOPLT (WHICH WAS COUPLED IN APCH MODE) AND THE AUTOTHROTTLES IN ANTICIPATION OF A POSSIBLE AVOIDANCE MANEUVER. WITHIN ABOUT ANOTHER 30 SECONDS TWR TOLD US TO CLB 500 FT; AND ALMOST SIMULTANEOUSLY TCASII GAVE US AN RA TO CLB. I CLBED ABOUT 500 FT AND MAINTAINED VISUAL CONTACT WITH THE TFC; WHICH PASSED TO OUR L (ABOUT 1/2 - 1 MI) AND BELOW US (ABOUT 500-800 FT). I THEN RESUMED THE APCH (FROM ABOUT 3 MI OUT); AFTER RECEIVING A 'CLR OF CONFLICT' FROM TCASII AND VERBAL CLRNC FROM TWR. THE REST OF THE APCH AND LNDG WERE UNEVENTFUL. SUPPLEMENTAL INFO FROM ACN 418645: THERE WERE SEVERAL TCASII TARGETS OVER BOEING FIELD JUST N OF SEA. TA BECAME AN RA ADVISING 'DSND; DSND NOW.' CAPT PUSHED NOSE OVER AND AT 1200 FT MSL RECEIVED TCASII 'MONITOR VERT SPD' CALL. JUMP SEAT RIDER CALLED OUT TFC AT 10 O'CLOCK. CAPT LOOKED OUT TO SEE PIPER ACFT PASS OFF L WING 400 FT HIGH; 1/4 - 1/2 MI AWAY. ACFT BEHIND US ALSO HAD CONFLICT WITH SAME ACFT; CAUSING A TCASII RA TO CLB. WE BOTH HAVE FILED COMPANY AVIATION SAFETY RPTS AND NASA RPTING FORMS. INVESTIGATION OF CONFLICT REVEALED SMA HAD DEPARTED BOEING FIELD AND DID NOT FOLLOW DIRECTIONS HE WAS GIVEN; THEREBY CAUSING CONFLICT WITH ACFT ON FINAL RWY 16R SEA TFC.