LESS THAN STANDARD SEPARATION BETWEEN 2 ACR ACFT. PLT DEVIATION OR OPERATIONAL ERROR.

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CLRNC WAS GIVEN TO DSND AND CROSS A POINT 50 MI SW OF KENTON VOR ON J191 AT FL270 TO MAINTAIN FL240. DURING THE DSCNT WE WERE TOLD OF OPP DIRECTION TFC AT FL260. AT THIS POINT I MENTIONED TO THE CTLR THAT THE CLRNC SOUNDED ALMOST AS IF WE HAD BEEN CLRED THROUGH SOMEONE ELSE'S ALT. HE GAVE US THE TFC POS AND DISTANCE AND THEN EXPLAINED THAT WITH HIS EXPERIENCE JUDGING SPDS; DISTANCE AND DSCNT RATES; THE TFC WOULD BE WELL PAST US PRIOR TO ANY ALT CONFLICTION. I SIGHTED THE ACFT PASS BELOW US AND SLIGHTLY TO THE LEFT; WELL CLR BOTH HORIZ AND VERT. WE COMPLIED WITH THE XING DISTANCE AND ALT RESTRICTIONS AS CLRED; BASED UPON OUR NAV EQUIP; AND CONTINUED TRIP AS NORMAL. AT NO TIME WERE WE ASKED TO CORRECT OR ALTER OUR HDG OR ALT TO AVOID A CONFLICTION DUE TO EITHER EQUIP OR HUMAN ERROR. THIS IS BASED ON MY BEST RECOLLECTION OF THE CLRNC OR EVENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: PIC SAYS THAT CTLR DID NOT INDICATE ANY PROB AT TIME OF INCIDENT; BUT A FEW DAYS LATER HE HEARD FROM ANOTHER SOURCE THAT THERE HAD BEEN A CONFLICT BTWN THE 2 ACFT WHICH SET OFF THE QAP AT ZDC. HE WAS ADVISED BY PLT ORGANIZATION REP TO FILE ASRS RPT. HIS RECOLLECTION OF DSCNT IS THAT THEY REACHED FL270 AND THE 50 MI SW OF ENO VOR POINT AT ABOUT THE SAME TIME AND IT RESULTED IN A CONTINUOUS DSCNT TO FL240. THEY SAW THE OPP DIRECTION TFC ABOUT SAME TIME THEY WERE APCHING FL270. SAYS THAT HE HAS BEEN TOLD THAT ZDC INVESTIGATION SHOWS THAT ACFT WERE 3.5 MI LAT WHEN AT SAME ALT. THEIR RATE OF DSCNT WAS NOT EXCESSIVE. COPLT FLYING MAY HAVE CALCULATED 50 MI POINT USING OMEGA RATHER THAN VOR/DME; AND THIS COULD RESULT IN A FEW MI DIFFERENCE.