FLC TAKES WRONG ACFT FOR DISPATCH.

Narrative

DURING FLT PLANNING FOR DEP; I SIGNED A RELEASE PROVIDED BY MY COMPANY DISPATCHER FOR B757 ACFT #XX34. WE BOARDED THE ACFT; WHICH WAS THE ONLY B-757 THAT WE COULD SEE. ALL ONBOARD WERE BOARDED AND READY FOR THE FLT FROM ANC TO DEN INCLUDING FLT ATTENDANTS; PAX; BAGGAGE; AND FUEL--ALL AT THE PROPER GATE. WE HAD TROUBLE GETTING INFO FROM ACARS; SO WE GOT OUR ATC CLRNC OVER THE RADIO. WT MANIFEST HAD TO BE GOTTEN OVER THE RADIO AND I DECIDED TO FIND WHAT THE ACAR'S PROB WAS AFTER DEP. ON CLBOUT; I COMMENTED HALF JOKINGLY THAT WE WERE PROBABLY IN THE WRONG ACFT. BECAUSE IT WAS DARK; WE TURNED UP SOME LIGHTS AND REALIZED THAT WE WERE IN ACFT #XX39 -- SURPRISE! WE TRIED IN VAIN GO CALL OUR DISPATCHER; SO CALLED THE ANC OPS OFFICE AND HAD THEM TELL OUR DISPATCHER. WITHIN MINS WE HAD NEW PAPERWORK FOR OUR AIRPLANE. THE TRAP FOR US WAS: EARLY AM DEP; ONLY B-757 IN SIGHT/AND ON CORRECT GATE; ETC. I AM STILL NEW TO ACARS OP; HAVING COME FROM B-727 WITH SO OPERATING IT. WE NEGLECTED TO CHK THE ACFT NUMBER TO PAPERWORK WHEN BOARDING (PROBABLY BECAUSE OF PAST RELIABILITY IN THIS SIT). SUGGESTIONS: INSTEAD OF AN ACARS MESSAGE LIKE E PN/FLT; WHY NOT 'WRONG ACFT'! MAYBE WE NEED A CHKLIST ITEM (OH NO!) TO CHK ACFT NUMBER. WE PROBABLY WON'T DO THAT AGAIN; BUT HOPEFULLY WE CAN HELP OTHERS TOO!