ACR MLG UNAUTH LNDG AT SEA FROM CAT II APCH.

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FLT WAS FLOWN BY CAPT. THE ILS RWY 16R CAT II APCH WAS BRIEFED. SEA APCH ISSUED THE FOLLOWING CLRNC. 'COMPANY IDENT; TURN RIGHT HDG 130; MAINTAIN AT OR ABOVE 3000' TILL ANVIL; CLRED ILS 16R APCH; TWR AT ANVIL.' I READ BACK THE CLRNC AS FOLLOWS. 'COMPANY IDENT CLRED ILS 16R APCH; HDG 130; AT OR ABOVE 3000' TILL ANVIL.' AT THAT POINT I WAS TUNED TO SEA VOR. THE CAPT TUNED TO THE ILS. WE INTERCEPTED THE LOC JUST OUTSIDE ANVIL. I TUNED AND IDENTIFIED THE ILS. THE CAPT COMMANDED GEAR DOWN AND I CONCLUDED THE FINAL DSCNT CHKLIST. WE CROSSED PARKK OUTER LOCATOR; CAPT ANNOUNCED TARGET APCH SPD. AT THAT POINT I WAS A BIT DISTR BY THE CONSTANT AURAL CODE OF THE OM. THE MARKER SWITCHES WERE NOT SILENCED BECAUSE OF THE NEED TO HEAR THE IM TO DETERMINE MINIMUMS FOR THE APCH. THAT DISTR PLUS THE TOTAL CONCENTRATION I WAS DEVOTING TO THE APCH WAS ENOUGH TO CAUSE ME TO DEVIATE FROM SOP AND NOT CONTACT TWR. HOLDING SHORT OF 16L I ADVISED. 'HOLDING SHORT 16L.' THE ANSWER WAS; 'THIS IS SEA APCH.' THAT WAS THE FIRST I REALIZED MY ERROR. I IMMEDIATELY SWITCHED TO TWR AND RECEIVED CLRNC TO CROSS 16L. CONCLUSION/SOLUTION. IN ANALYZING THIS EVENT; I MUST TAKE FULL RESPONSIBILITY FOR NOT ADHERING TO PROCS; BUT I ALSO FEEL THAT THIS IS A WEAK SPOT IN THE APCH CLRNC PROC. THROUGHOUT THE ENTIRE FLT; PLTS ARE DIRECTED BY ATC WHEN TO CHANGE FREQ AND THE TRANSFER IS MADE IMMEDIATELY. YET ON SOME APCHS; THE FINAL SWITCH TO TWR IS NOT COMMANDED BY ATC BUT IS TRIGGERED BY SOME OTHER EVENT. IN THIS CASE THE 11 DME FIX OFF OF SEA ILS. DURING A PERIOD OF HIGH CONCENTRATION AND WORKLOAD SUCH AS A CAT II APCH; THE PLT MIGHT NOT 'KEY' INTO A PASSIVE FREQ CHANGE DEVICE. THAT WAS MY PROB ON THIS APCH. MY SOLUTION IS; ALL FREQ CHANGES FROM APCH TO TWR SHOULD BE INITIATED AS POSITIVE COMMANDS BY APCH CTL. 'IDENT; CONTACT TWR 119.9.' THIS MIGHT NOT BE A WORKABLE SOLUTION FROM THE ATC VIEWPOINT; BUT MY MISTAKE WOULD HAVE BEEN AVOIDED UNDER SUCH A PROC.