ACR MAKES CAT II APCH. AFTER LNDG DISPATCH SAYS APCH NOT AUTHORIZED.

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AFTER DEPARTING DCA FOR GSO THE WX BEGAN TO DETERIORATE. RVR WAS FLUCTUATING BTWN 1100 AND 1600. THERE WERE NO NOTAMS LISTED ON THE ATIS NOR ON THE WX HARD COPY PROVIDED BY OPS AT DCA. WHEN GSO APCH INFORMED US THE RVR WAS DOWN TO 1100 FT; THE CAPT TOLD THEM WE NEEDED 1200 FT FOR THE RWY 23 CAT II ILS APCH. WE WERE ISSUED HOLDING INSTRUCTIONS AT THE OM. PRIOR TO ENTERING THE HOLD THE RVR WENT TO 1200 FT AND WE WERE 'CLRED FOR THE APCH.' WE PERFORMED A MONITORED APCH WITH THE CAPT WATCHING 'OUTSIDE' WHILE I MONITORED 'INSIDE.' WHEN I CALLED OUT 100 FT ABOVE MINS THE CAPT CALLED THE RWY IN SIGHT TOOK CTL OF THE ACFT AND MADE A NORMAL; UNEVENTFUL LNDG. WE WERE NOTIFIED BY THE DISPATCHER THE APCH WAS 'NOT AUTHORIZED.' CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. CALLBACK MADE TO DETERMINE ON WHAT BASIS APCH NOT AUTHORIZED. RPTR STATES HE DOES NOT KNOW. CAPT GOING TO CHK BUT HAS NOT DONE SO. A NOTICE POSTED ON CREW ROOM BOARD REF NOT AUTHORIZED; BUT SEEN AFTER THE FACT. NO NOTAM ISSUED. RPTR ASKED THROUGH HIS CHAIN OF COMMAND BUT RECEIVED NO REAL ANSWER. IT APPEARS TO BE THE GREAT MYSTERY. NOTICE NO LONGER ON CREW BOARD.