LGT FLIES WRONG SID FROM SEA.

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PRIOR TO BLOCK-OUT; WE RECEIVED OUR PRE-DEP CLRNC ON ACARS. THE SID SPECIFIED WAS THE 'MOUNTAIN 2 DEP (VECTOR);' MAR/93. SINCE I HAD NOT BEEN TO KSEA IN QUITE SOME TIME; THE DAY BEFORE; I LOOKED AT THE SEATTLE COMMERCIAL CHARTS AND DETERMINED THAT A 'SEATTLE 2 DEP (VECTOR);' MAR/93; WAS MOST LIKELY. WHILE CHKING THE PRE-DEP CLRNC; THE CAPT AND I WERE WORKING A FUELING DISCREPANCY PROB. EVEN THOUGH I PHYSICALLY READ 'MOUNTAIN 2;' I WAS MENTALLY LOCKED-IN TO A 'SEATTLE 2.' THE FUELING DISCREPANCY RESOLVED; WE BLOCKED OUT ON TIME AND PROCEEDED DIRECTLY TO THE END OF RWY 16L FOR TKOF. AS WE WERE CLRED ONTO THE RWY FOR TKOF; I BRIEFED 'STRAIGHT AHEAD TO 9000 FT (OUR INITIAL CLRED ALT) FOR VECTORS;' STILL MENTALLY LOCKED-IN TO A 'SEATTLE 2 DEP.' AFTER CLEANING THE AIRPLANE UP AT 1000 FT AGL; I ADVISED THE CAPT THAT I WOULD MAINTAIN 210 KTS UNTIL WE WERE TURNED EBOUND. AT APPROX 8000 FT MSL; THE DEP CTLR ASKED IF WE HAD BEGUN OUR TURN YET. I BELIEVE THE CAPT REPLIED THAT WE HAD NOT RECEIVED A VECTOR YET. AT THAT POINT; THE CTLR ASKED IF WE HAD NOT BEEN CLRED FOR A 'MOUNTAIN 2 DEP.' WE QUICKLY CHKED THE PRE-DEP CLRNC AND ADVISED ATC THAT WE HAD. HE THEN GAVE US A 030 DEG VECTOR TO JOIN J-90 AND CLB TO 17000 FT; I BELIEVE. LATER DURING THE CLBOUT; THE CAPT AND I DISCUSSED THE EVENT. THE 'MOUNTAIN 2' REQUIRES A L TURN TO 070 DEG HDG AFTER XING THE SEA 158 DEG/5 DME AT OR ABOVE 3000 FT MSL. THE 'SEATTLE 2' REQUIRES FLYING THE 158 DEG RADIAL OF SEA FOR EXPECTED VECTORS. WE WERE WELL ABOVE 3000 FT AT 5 DME DUE TO OUR 210 KIAS CLB SPD. WHILE NOT SPECIFICALLY NOTING OUR DME WHEN ATC ASKED IF WE'D BEGUN OUR TURN YET; I ASSUME IT WAS ABOUT 6-8 DME; AS WE WERE CLBING QUITE RAPIDLY ANTICIPATING EITHER A LEVEL-OFF AT 9000 FT OF FURTHER CLRNC (VECTORS AND/OR ALT). POSSIBLE CAUSES: 1) BECOMING MENTALLY LOCKED-IN TO A PRECONCEIVED ATC CLRNC. 2) NOT TRULY READING AND COMPREHENDING THE ACTUAL CLRNC. 3) NOT BRIEFING THE DEP CLRNC BY NAME. 4) SECOND LEG (MY FIRST FLYING) OF A 2- DAY; 2-LEG TRIP; FOLLOWING 1 MONTH VACATIONS FOR BOTH PLTS. 5) MINOR FUELING DISTR PRIOR TO DEP WHILE PRE-DEP CLRNC WAS RECEIVED AND REVIEWED.