C210 PLT HAS AN NMAC DURING DSCNT TO SMO.

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RPTR; IN CESSNA CENTURION; WAS ON A VFR FLT WITH FLT FOLLOWING FROM SBP TO SMO. AT TIME OF EVENT; I HAD RECENTLY BEEN HANDED OFF FROM MUGU APCH TO SOCAL LAX W SECTOR ON 125.2. I WAS BELOW LAX CLASS B AIRSPACE DSNDING EBOUND ALONG COAST LINE; WITH LOWER CLASS B AIRSPACE TO MY R (S). AS I APCHED THE MALIBU CANYON AREA MY TIS SHOWED AN ACFT AT A SLIGHTLY LOWER ALT AND AHEAD GOING SBOUND (FROM MY L TO MY R). I VISUALLY DETECTED THE ACFT (A PIPER CHEROKEE) AS IT TURNED EBOUND AT APPROX MY 1 O'CLOCK POS AND 1 MI OR SO. IT WAS WELL DEFINED AGAINST THE OCEAN. I WAS OVERTAKING IT AND PLANNED TO PASS IT TO ITS L. THERE WAS ADEQUATE TERRAIN CLRNC TO DO SO; EVEN IN DSCNT. SHORTLY AFTER I SAW THIS ACFT; THE CTLR CALLED IT AND ANOTHER ACFT. I NEVER SAW THIS SECOND ACFT; BUT IT PASSED US. A FEW MOMENTS LATER; MY TIS SUDDENLY DISPLAYED ANOTHER ACFT AT MY ALT AHEAD AND COMING TOWARD ME AND THE CHEROKEE. AT THAT POINT I HAD DSNDED ALMOST TO THE SAME ALT AS THE CHEROKEE I WAS OVERTAKING. THE CTLR CALLED THIS TFC ABOUT THE SAME TIME THAT I DETERMINED ITS ALT. I PUSHED THE NOSE DOWN TO DSND THROUGH ITS ALT (AND THE CHEROKEE'S); TURNING L SLIGHTLY TOWARD THE TERRAIN TO REMAIN CLR OF THE CHEROKEE. THE OPPOSITE-DIRECTION ACFT PASSED R OF AND VERY CLOSE TO THE CHEROKEE -- 50 FT OR LESS HORIZONTALLY. IT APPEARED TO BE AT THE SAME ALT; BUT BECAUSE I WAS JUST SLIGHTLY BELOW AND BEHIND THE CHEROKEE TO THE L; THE OPPOSITE-DIRECTION ACFT WAS PROBABLY JUST SLIGHTLY ABOVE; BUT NOT MUCH. MY GUESS WAS THAT ALL 3 ACFT WERE WITHIN 150 FT OF EACH OTHER. WE WEREN'T TOO FAR FROM A SITUATION WHERE WE ALL ARRIVED AT THE SAME POINT AT THE SAME TIME. THE CHEROKEE PLT APPARENTLY DIDN'T SEE THE OPPOSITE-DIRECTION ACFT (WHICH I COULDN'T IDENT) UNTIL IT WAS PASSING HIM; BUT HE REACTED BY BANKING AWAY TO THE L AND TOWARD ME; WHICH I ANTICIPATED AND ALSO TURNED L TOWARD THE TERRAIN. I WAS ABEAM OF THE CHEROKEE AT THAT POINT; AND HE APPEARED TO BE STARTLED WHEN HE SAW ME. I RECEIVED A TERRAIN WARNING AS A RESULT OF THIS TURN AND CONTINUING DSCNT; BUT THERE WAS NO ACTUAL DANGER. THE CHEROKEE LANDED AT SMO SHORTLY AFTER ME. I TRIED TO FIND THE PLT AFTER SECURING MY ACFT BUT WAS UNABLE TO DO SO.