A B737 PILOT APCHING LAX NOTES THAT B737-300 AND B737-700 MCP (MODE CONTROL PANEL) DIFFERENCES CAN PERMIT A PILOT FLYING THE -300 TO THINK THE LOC IS ACQUIRED WHEN IT IS NOT.

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WHILE FLYING THE SEAVU 1 ARR INTO LAX (RWY 25L) IN A -300 WE WERE CLRED FOR THE ILS APCH TO RWY 25L AFTER REACHING FUELR (D26.4 LAX). I WAS THE PF AND TOGGLED FROM NAV TO HSI ON THE CAPT'S HSI SWITCH. I THEN SELECTED LOC ON THE AUTOPLT AND DESELECTED LNAV SO THAT THE ACFT WOULD TRACK THE LOC INBOUND TO RWY 25L UPON REACHING THE FUELR INTXN. AT APPROX 7-9 MI; I TOGGLED THE HSI SWITCH MOMENTARILY TO NAV TO XCHK OUR POS RELATIVE TO THE LNAV COURSE AND AT THAT TIME WAS DISTR BY A TFC CALL. WHILE TRYING TO VISUALLY ACQUIRE THE TFC I FAILED TO NOTICE THAT THE AUTOPLT WAS NOT COUPLED TO THE LOC WHEN I RETURNED THE HSI SWITCH BACK TO 'HSI.' I ALSO FAILED TO NOTICE THAT THE ACFT HAD DRIFTED N OF CTRLINE. WHEN I DID NOTICE AND STARTED TO MAKE THE CORRECTION BACK TO THE LOC CTRLINE FOR RWY 25L; TWR CALLED AND ASKED IF WE WERE TRACKING THE LOC INBOUND FOR RWY 25L. WE ACKNOWLEDGED THAT WE WERE N OF CTRLINE AND WERE CORRECTING BACK TO THE CTRLINE. WE WERE RE-ESTABLISHED ON THE CTRLINE AT APPROX 2500 FT AGL (PRIOR TO THE LIMMA APCH FIX) WITH THE CORRECT SPD AND CONFIGN AND LANDED ON RWY 25L. WHEN FLYING A -300; IT IS IMPERATIVE THAT THE CREWS BE EXTRA VIGILANT THAT THE LNAV/NAV CONFIGN AND OP IS SUFFICIENTLY DIFFERENT FROM THE -700 AND THAT AIRSPACE DEV'S OF THIS SORT ARE MORE LIKELY TO OCCUR. WE WERE FLYING EXCLUSIVELY -700 ACFT FOR THE PREVIOUS 2 DAYS AND THEN FLEW A -300 ON THE LAST DAY OF THE PAIRING. IN MY ATTEMPT TO GATHER AS MUCH POS INFO IN THE -300 THAT IS AVAILABLE AT A GLANCE IN THE -700; I FAILED TO NOTICE THAT THE AUTOPLT WAS NOT TRACKING THE LOC AS I HAD EXPECTED. IN THE -700 ACFT; CREWS CAN SIMULTANEOUSLY MONITOR BOTH WITHOUT PHYSICALLY REPOSITIONING ANY SWITCHES. CREWS NEED TO PAY EXTRA ATTN TO THE 'COUPLED STATUS' OF THE AUTOPLT WHEN SWITCHING BACK AND FORTH FROM HSI TO NAV; PARTICULARLY ON APCH.