ACR HVT TKOF ABORT WHEN FO'S SEAT SLID FULL BACK AWAY FROM THE CTLS.

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OUR ACFT WAS ON TKOF ROLL AT DEN; RWY 35R. AT ABOUT 100- 110 KIAS; THE FO'S SEAT CAME LOOSE ON THE TRACK AND SLID FULL AFT. FO WAS FLYING AT THE TIME; COULD NOT REACH RUDDER PEDALS. AS ACFT INITIATED A SLIGHT R DRIFT; CAPT ELECTED TO REJECT TKOF; TOOK CTL OF THE ACFT AND EXECUTED AN UNEVENTFUL REJECT. ACFT REQUIRED 1 HR; 40 MINS GND TIME FOR BRAKE COOLING. MAINT TALKED WITH COMPANY MAINT AT LOUISVILLE AND REQUESTED INTERP OF REGS. DECISION WAS MADE TO POS SEAT TO FIT FO'S FLYING POS AND PIN/LOCK SEAT IN PLACE. ITEM WAS DEFERRED IN LOGBOOK. CAPT AND CREW WERE BRIEFED ON STATUS AND AGREED WITH DECISION. UPON ARR AT LOUISVILLE AFTER UNEVENTFUL FLT; WE LEARNED THAT DISPATCH HAD BEEN INVALID DUE TO INCORRECT MAINT DEFERRAL. COMPANY ADMITTED IT WAS A COMPANY MISTAKE; THAT CREW HAD NOT DONE ANYTHING WRONG AND HAD ACTED IN GOOD FAITH WITH UNDERSTANDING DISPATCH WAS LEGAL. COMPANY DIDN'T BELIEVE ANY PROBLEM EXISTED OR ANY FALLOUT WOULD RESULT BUT RECOMMENDED FILING OF THIS RPT. ESSENTIALLY A NON EVENT FROM AIRCREW PERSPECTIVE. SUPPLEMENTAL INFO FROM ACN 189502: I ASSUMED CTL AND CALLED FOR A REJECTED TKOF THINKING WE HAD LOST AN ENG.