A DHC8-200 WAS DISPATCHED WITH THE CREW OXYGEN OUT OF LIMITS. ITEM DEFERRED WITH LIMITED ALT RESTR. THE REFERENCE USED FOR DEFERRAL AUTH WAS INCORRECT.

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CREW CALLED ME IN MAINT CTL; SAID THEY WERE LATE AND NEEDED A DEFERRAL ON CREW FIXED OXYGEN. IT WAS AT 900 PSI IN ZZZ1. THEY STATED THEY HAD NO PROB FLYING BELOW 10000 FT TO ZZZ2 AND BACK. I CHKED WITH DISPATCH AND THEY CONCURRED. PLT STATED HE WASN'T SURE WHERE THE CHART WAS AT; AND I ALSO WAS BLANK ON WHERE TO FIND IT. I DID FIND IT IN THE BACK OF THE MEL BOOK IN THE APPENDIX; MISREAD THE CHART; ISSUED A DEFERRAL AND FLEW THE ACFT ON A ROUND TRIP ZZZ-ZZZ2-ZZZ1 BELOW 10000 FT. IN ZZZ ON RETURN TRIP; HAD ZZZ1 SVC THE OXYGEN AND CLR THE DEFERRAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE MEL WAS NOT USED AS THE REFERENCING AUTH FOR DEFERRING THE CREW OXYGEN SYS. THE RPTR ADMITS THE ERROR IN USING THE SPECIAL ITEM LIST AS THE DEFERRAL AUTH. THE SPECIAL ITEM LIST IS FOR ADMINISTRATIVE USE ONLY; AS IT LISTS SPECIAL ITEMS THAT ARE OPEN OR A MAINT ITEM THAT MAKES THIS AIRPLANE DIFFERENT FROM THE REST OF THE FLEET. AN UNCOMPLETED MODIFICATION; PAX SEAT CONFIGN; ARE ITEMS THAT WOULD BE LISTED IN THE SPECIAL ITEM LIST.