CONFLICT BTWN A DSNDING C208 AND AN UNKNOWN GA ACFT NEAR THE TFC PATTERN ENTRY POINT 2 NM NW OF ONT; CA.

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I WAS THE PIC OF A SCHEDULED PART 135 CARGO FLT. I DEPARTED BFL ON AN IFR FLT PLAN WITH ONT AS MY DEST. UPON CHKING IN (ON TOP 7500 FT DIRECT POM VORTAC) WITH SOCAL TRACON ON 125.5; I CANCELED MY IFR AND CONTINUED WITH RADAR ADVISORIES. THE SECTOR CTLR ACKNOWLEDGED MY IFR CANCELLATION; AND GAVE ME INSTRUCTIONS TO 'ENTER THE N DOWNWIND FOR THE RWY 26 COMPLEX; AT OR ABOVE 4000 FT.' I READ THIS INSTRUCTION BACK; AND STARTED A VFR DSCNT TO 4000 FT MSL. AFTER A FEW MINS; THE CTLR CLRED ME TO 'DSND AT OR ABOVE 3500 FT.' I ACKNOWLEDGED THIS WITH A READBACK. I WAS THEN TOLD TO CONTACT ONT TWR ON 120.6 MHZ. I ACKNOWLEDGED THIS INSTRUCTION WITH A READBACK. I THEN CONTACTED ONT TWR. I ADVISED THEM I WAS ON 'R DOWNWIND; RESTR.' THE LCL ONT CTLR ACKNOWLEDGED MY CALL WITH THE INSTRUCTION TO 'CONTINUE YOUR DSCNT; NOT BELOW 2500 FT.' I READ THIS INSTRUCTION BACK 'AT OR ABOVE 2 POINT 5.' DURING MY DSCNT TO 2500 FT; I OBSERVED CONFLICTING TFC GOING OPPOSITE DIRECTION; APPROX 150 FT HIGHER; AND 500 FT TO MY L SIDE. I BELIEVE THIS ACFT WAS ON APCH TO THE NEARBY POC ARPT. I THOUGHT THAT THE SEPARATION WAS INADEQUATE; SO I ASKED ONT LCL CTLR TO CONFIRM MY ALT ASSIGNMENT AS 'AT OR ABOVE 2500 FT.' THE CTLR SAID 'NO; 3500 FT.' HE THEN CLRED ME TO LAND. I AM SURE I WAS GIVEN THE 2500 FT RESTR. BUT; IT IS POSSIBLE THAT I MISHEARD MY INSTRUCTIONS; AND HE MISHEARD MY READBACK.