DH8A CREW HAD NMAC WITH A C337 IN ABE CLASS E AIRSPACE.

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ON OUR FLT FROM SWF DIRECT PHL; WE WERE BEING VECTORED BY ABE APCH FOR SPACING INTO PHL. WE WERE GIVEN A 270 DEG HDG AT 8000 FT. AFTER A FEW MI WE WERE GIVEN A DSCNT TO 5000 FT. THROUGH 6500 FT WE WERE GIVEN A TURN TO 180 DEGS. TCASII SHOWED TFC BELOW US AT 4500 FT. WE WERE LOOKING FOR THAT TFC WHEN WE ROLLED OUT ON A 180 DEG HDG AT 5000 FT (230 KTS). JUST AS WE ROLLED OUT AND LEVELED; I SAW A SKY MASTER AT 12 O'CLOCK POS AT OUR ALT HDG FOR US. AS EVASIVE ACTION OF A DIVING R TURN HAD US PASS THIS ACFT 100 FT VERT AND 1000 FT HORIZ. WE THEN GOT AN RA BECAUSE OF THE TFC AT 4500 FT. I THEN CLBED AND TURNED BACK TO A 180 DEG HDG. RPTED THE NEAR MISS TO ABE. AFTER THE RPT; THEY SAID THE SKY MASTER (084 IDENT) DID NOT HAVE HIS XPONDER ON. THEY TRACKED THE PRIMARY TARGET UNTIL THEY LOST HIM N OF THE ETX VOR. COMPANY TFC 5 MI BEHIND US CONFIRMED THE ACFT WAS AT 5000 FT HDG N. THE ACFT NEVER TOOK EVASIVE ACTION AND SEEMED TO BE FLYING V29 (VFR) AT THE ALT OF 5000 FT. I WOULD REALLY LIKE TO KNOW WHAT THIS GUY WAS THINKING. BUT I DOUBT HE WAS PAYING ATTN; OTHERWISE HE WOULD HAVE TURNED. IT WAS THAT CLOSE!