B737 ENCOUNTERS OPPOSITE DIRECTION TFC NOT AT PROPER ALT DURING CRUISE IN ZNY OCEANIC AIRSPACE.

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WE WERE FLYING NBOUND UP A-300 WHEN WE VISUALLY SIGHTED AN ACFT SBOUND AND TURNED ON OUR LNDG LIGHTS. AS THE DISTANCE BTWN US CLOSED; THE FO NOTICED TCASII INDICATED ONLY 500 FT ALT SEPARATION. BOTH ACFT INITIATED A R-HAND TURN TO INCREASE SEPARATION AND TCASII ISSUED A TFC ALERT. A FEW SECONDS LATER THE TCASII ISSUED AN RA AND INSTRUCTED US TO DSND. AFTER 'CLR OF CONFLICT' WAS ANNOUNCED I NOTICED ON TCASII THAT THERE WAS 1300 FT SEPARATION WITH OTHER ACFT (NOW BEHIND US). I HAD MAINTAINED FL280. I ATTEMPTED TO CONTACT OTHER ACFT ON COMMON FREQ 130.55 WITHOUT SUCCESS. SHORTLY AFTER WE ANSWERED SELCAL TO VERIFY OUR FLT LEVEL AND RPTED LEVEL AT FL280. COMRDO INFORMED US THAT OTHER ACFT HAD RPTED THE 500 FT ALT SEPARATION. WE HAD ALREADY CHKED EACH OF OUR ALTIMETERS AND CONFIRMED FL280 AND 29.92 SET IN EACH OF THE 3 INSTS. APPROX 3-5 MINS LATER; WE SIGHTED ANOTHER SBOUND ACFT APCHING AND TURNED ON LNDG LIGHTS. TCASII INDICATED 1000 FT ALT SEPARATION; AND OTHER ACFT PASSED DIRECTLY OVERHEAD. WE INCLUDED THIS INFO IN OUR NEXT POS RPT. I BELIEVE THE SBOUND ACFT WE PASSED AT 500 FT SEPARATION WAS 480 FT BELOW HIS ASSIGNED ALT OF FL290. AFTER PASSING HIM WE RECHKED ALL OF OUR INSTS AND DID NOT HAVE TO ADJUST OUR ALT WHEN THE SECOND ACFT PASSED OVERHEAD. I THEN CHKED MY PAPERWORK AND FOUND THE NYC ALT SETTING WAS 30.43 AT TIME OF OUR DEP. SINCE A-300 IS MOSTLY USED SBOUND BY ACFT DEPARTING THE NEW YORK AREA; I SURMISE THAT OUR CONFLICTING TFC PROBABLY HAD THEIR ALTIMETER SET AROUND 30.43 INSTEAD OF 29.92 WHICH RESULTED IN AN ALT APPROX 480 FT BELOW FL290.