CHARTER SMT HAD CLOSE ENCOUNTER WITH UNKNOWN VFR LIGHT TWIN.

Narrative

ACFT WAS HANDED OFF TO READING APCH CTL OVER POTTSTOWN; LEVEL AT 4000'. AFTER CONTACT WITH READING; THE PLT REPORTED TO ME A NEAR MISS OVER POTTSTOWN. THE OTHER ACFT INVOLVED WAS UNKNOWN TO ME. THE ACFT INVOLVED; WERE BOTH IN PHILADELPHIA'S AIRSPACE AND SMT X WAS IFR ON THE ROUTE. SMT X REPORTED A GREEN AND WHITE TWIN; FROM THE NORTH AND HIS RIGHT. TOOK EVASIVE ACTION AND CAME WITHIN 200' OF SMT X. NO OTHER INFORMATION AVAILABLE. READING'S RADAR COVERAGE IN THIS AREA IS EXTREMELY POOR; ACFT MUST BE AT LEAST 4000' OR ABOVE TO GET A GOOD RETURN. READING FIELD ELEVATION IS 344. I DON'T HAVE PHILADELPHIA'S REPORT; BUT THIS IS A FAIRLY CONGESTED AREA; TOO BUSY FOR TFC ADVISORIES OR POSSIBLY NO XPONDER; ETC. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE OTHER ACFT INVOLVED IN THIS NEAR MIDAIR COLLISION WAS AN UNREPORTED VFR TWIN PASSING THROUGH THE AREA. PHL APCH CTL HAD NO INFO ON THE ACFT BUT DID SEE A 1200 SQUAWK JUST AFTER THE CHANGE OVER WAS MADE ON SMT X. REPORTER'S EXPERIENCE LEVEL WAS ASCERTAINED.