RAPID CABIN PRESSURIZATION LOSS LED TO EMER DSCNT IN B737-200 WITHOUT CLRNC.

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LOST CABIN PRESSURIZATION; CABIN RATE OF CLB INDICATOR WAS PEGGED AT PLUS 4000 FT. ASKED ATC FOR LOWER 3 TIMES THEN INITIATED EMER DSCNT TO 10000 FT. NEVER GOT A RESPONSE FROM ATC. DEVIATED FROM FL200 TO 10000 FT BECAUSE OF RAPID DEPRESSURIZATION. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: EXCESSIVE CABIN CLB LED FLC TO BELIEVE THAT THEY HAD LOST PART OF THE FUSELAGE. AN EMER DSCNT WAS REQUESTED BUT CLRNC WAS GIVEN TO SIMILAR SOUNDING CALL SIGN ('XZZ' INSTEAD OF XYZ'). FLC INITIATED FURTHER DSCNT IN VMC AND IN AREA OF ANTICIPATED DSCNT CLRNC. AFTER THE SIT WAS UNDER CTL AT 10000 FT; THE FLC WAS UNABLE TO ESTABLISH COM ON LAST ASSIGNED FREQ; SO THEY WENT TO APCH. FLT WAS EXPEDITED. AFTER LNDG IT WAS DISCOVERED THAT A CARGO DOOR SEAL HAD BLOWN LEAVING APPROX A 4 INCH GAP. MAINT REMARKED THAT THIS WAS A FAMILIAR PROB. RPTR (MONITORING PLT) STATED THAT COMPANY'S AVIATION PSYCHOLOGIST FOLLOWED-UP ON FLC'S REACTION TO THE EVENT -- EXTENSIVE QUESTIONNAIRE INVOLVED. SUPPLEMENTAL INFORMATION FROM ACN 307828: COULD NOT NOTIFY ATC SO SQUAWKED 7700.