ASSIGNED SID NOT FLOWN ON DEP FROM SEATTLE; WA. CREW CLAIMS CONFUSION OF FORMAT OF PRE-DEP CLRNC.

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ON CLBOUT FROM SEA ON RWY 341 DEG HDG PASSING APPROX 8000 FT AT ABOUT 14 DME; DEP ASKED IF WE HAD BEEN ASSIGNED A DEP AND INFORMED US THAT WE WERE TO HAVE TURNED TO 070 DEGS AT 8 DME. WE DISCOVERED THAT WE HAD MISSED OUR PRE-DEP CLRNC WHICH STATED 'MOUNTAIN 5' DEP -- WE MISTAKENLY INTERPED THIS AS 'MOUNTAINS' IE; MOUNTAIN WITH AN 'S' ON THE END AND NOT A '5.' WE FEEL TO ELIMINATE CONFUSION IN THE FUTURE 'MOUNTAIN 5' SHOULD BE SPELLED OUT 'MOUNTAIN FIVE.' SUPPLEMENTAL INFO FROM ACN 525763: ON CLBOUT FROM SEATAC ON RWY 34R; 341 DEG HDG; CLBING THROUGH APPROX 8000 FT MSL; THE DEP CTLR QUESTIONED US ABOUT OUR ASSIGNED SID; AND TOLD US THAT WE WERE SUPPOSED TO BE ON THE MOUNTAIN 5 (FIVE) SID THAT HAD A TURN TO 070 DEGS AT 8 DME (SEA). AFTER LEVELOFF; WE RECHKED THE PRINTED COPY OF THE PRE-DEP CLRNC AND SAW THAT WE HAD 'READ' THE SID ASSIGNMENT AS A WARNING -- 'MOUNTAINS' NOT AS THE 'MOUNTAIN 5' SID. THE 'MOUNTAIN 5' SID WAS NOT PART OF OUR FMC DATABASE. SPELLING OUT THE NUMBER WOULD HAVE ELIMINATED OUR CONFUSION. THERE WERE NO TFC CONFLICTS AND THE DEP CTLR DIDN'T SEEM CONCERNED. TO ADD TO IT ALL; TWR ASKED FOR A 'BASES' AND 'TOPS' RPT ON OUR DEP. THIS WAS NOT A FACTOR; BUT COULD HAVE BEEN IF THE 'TOPS' WOULD HAVE BEEN HIGHER. OTHER ARPTS THAT I HAVE FLOWN OUT OF; SPELL OUT THE NUMBERS OF THE SID.