FLC MISTAKES PROPER ACFT TO BE FOLLOWING ON VISUAL APCH.

Narrative

WE WERE VECTORED N OF THE ARPT DUE TO TSTM IN THE VICINITY OF THE LOC BC TO RWY 19. WE WERE N OF THE TSTM HEADING EBOUND UNDER A LAYER OF CLOUDS. WE WERE TOLD THAT WE HAD TFC AT THE 2 O'CLOCK POS FOR 3 MI. I SAW THE TFC; AND MY CAPT ALSO SAID THAT HE SAW THE TFC. WE BOTH ACKNOWLEDGE THAT THE TFC WAS AT OUR 2 O'CLOCK POS. AT THIS POINT THE CAPT TOLD THE APCH CTLR THAT WE HAD TFC IN SIGHT. THE CTLR ASKED IF WE COULD FOLLOW THAT TFC. HE SAID WE COULD. WE WERE THEN TOLD TO FOLLOW THAT TFC AND THAT WE WERE CLRED FOR THE VISUAL TO RWY 19. WE FOLLOWED THE TFC -- THEN THE CTLR TOLD US THAT THE TFC WOULD BE SQUARING HIS BASE. AT THAT TIME WE SAW A BOEING 737 OFF OUR L SIDE 500 FT BELOW AND 1/2 MI AWAY. THE CTLR SAID THAT ACFT WAS OUR TFC AT OUR 9 O'CLOCK POS. THEY WERE SUPPOSED TO BE FOLLOWING HIM. MY CAPT REPLIED NO THAT WASN'T OUR TFC -- OUR TFC WAS ON FINAL AT THIS POINT. WE HAD A NEAR MISS WITH A 737. WE WERE NOT FOLLOWING THIS ACFT AS WE WERE TOLD TO FOLLOW TFC THAT HAD PREVIOUSLY BEEN CALLED TO US AT OUR 2 O'CLOCK POS; NOT OUR 9 O'CLOCK POS. THIS WAS UNINTENTIONAL FOR WE BELIEVED THAT WE WERE IN FACT FOLLOWING THE RIGHT TFC.