B737-300 WAS DISPATCHED WITH #2 SLAT ACTUATOR ACCESS PANEL NOT INSTALLED.

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ON SEP/MON/05 WE WENT TO ZZZ TO REPLACE THE #2 SLAT. AFTER WE ARRIVED IN ZZZ; WE REALIZED WE ALSO HAD TO REPLACE THE L-HAND OUTBOARD FORE FLAP ALSO. WE R&R'ED SLAT AND FLAP. I THEN PROCEEDED TO THE FLT DECK TO OPS CHK FLAP AND SLAT. MECH ON GND VERIFIED EVERYTHING WAS GOOD. I RAISED FLAPS AND SLATS FULL UP AND WENT DOWN TO INST LEADING EDGE PANELS ON THE WING. THE FLAPS AND SLATS NEVER CAME DOWN AGAIN. OPS CAME AND TOLD US TO MOVE AIRPLANE TO THE GATE ASAP. WHILE WE WERE STILL INSTALLING PANELS; TUG AND TOW BAR WERE HOOKED UP BY RAMPERS. WE FINISHED INSTALLING PANELS AND TOWED THE AIRPLANE TO THE GATE. WE FLEW BACK TO ZZZ2. DURING FLT; WE REALIZED NONE OF US INSTALLED THE SLAT ACTUATOR ACCESS PANEL ON THE SLAT. SINCE THE FLAPS NEVER CAME DOWN; WE DID NOT SEE THAT THE PANEL WAS MISSING. WHEN WE ARRIVED IN ZZZ2; WE NOTIFIED MAINT CTL AND ENTERED DISCREPANCY INTO LOGBOOK; WHERE THE PANEL WAS INSTALLED. IN MY OPINION; THE CONTRIBUTING FACTORS WERE MAINT CTL; OPS AGENTS PRESSURING US TO HURRY AND FINISH AND TELLING US HOW MANY FLTS THE AIRPLANE HAD MISSED AND ALSO UNEXPECTED MAINT.