A B737-300 HAD OPEN DEFERRED ITEM #1 ENG HYD DEPRESSURIZATION SYS WAS INOP. MEL REQUIRED DEPRESSURIZATION OVERRIDE PIN REQUIRED FOR PUSHBACK. PIN NOT USED!

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AS WE MET THE INBOUND CREW; WE SAW THAT OUR ACFT HAD AN MEL 29-10 FOR A '#1 ENG SYS 'A' DEPRESSURIZATION FUNCTION INOP.' WE WERE BRIEFED BY THE INBOUND CREW THAT THEY PUSHED BACK FIRST -- HAD THE TOW BAR RELEASED; AND THEN STARTED BOTH ENGS. AS I CHKED THE MEL; I SAW AN ADDITIONAL REQUIREMENT FOR THE NOSEWHEEL STEERING OVERRIDE PIN TO BE INSTALLED PRIOR TO PUSH AND THE #1 ENG TO BE STARTED AFTER TOW BAR RELEASE. I CALLED ZZZ1 MAINT OUT TO ASK THEM TO SHOW ME (AND THE GND PUSHBACK PERSON) HOW TO INSTALL THE OVERRIDE PIN. THE MAINT PERSON SHOWED US WHERE THE OVERRIDE WAS LOCATED; BUT INFORMED US THAT MAINT NO LONGER USED THAT PIN AND THAT THEY DIDN'T EVEN HAVE ANY OF THEM THERE. I THEN CALLED MAINT IN ZZZ2 AND TALKED TO MAINT CTL. I EXPLAINED TO HIM THAT THE MEL REQUIRED THE OVERRIDE PIN TO BE INSTALLED PRIOR TO PUSH. HE SAID THAT IT WAS NOT REQUIRED TO PIN THE OVERRIDE AS LONG AS WE WAITED TO START THE #1 ENG UNTIL AFTER TOW BAR RELEASE; AND THAT IT WAS NOT COMPANY PROC TO USE THE PIN. THE ACTUAL WRITE-UP FOR THE SYS 'A' HYD DEPRESSURIZATION STATED THAT 'THE SYS DID DEPRESSURIZE AFTER A FEW MINS.' IN FACT; THE 'A' SYS WAS DEPRESSURIZED WHEN I FIRST ENTERED THE COCKPIT. BASED ON THE FACTS THAT: 1) THE 'A' SYS ACTUALLY WAS DEPRESSURIZED FOR PUSH; 2) ZZZ MAINT SAID THAT THEY DID NOT EVEN HAVE A PIN; AND 3) ZZZ2 MAINT CTL STATED THAT IT WAS NOT COMPANY PROC TO INSTALL THE PIN -- I PUSHED BACK AND STARTED BOTH ENGS ONLY AFTER CONFIRMING THAT THE TOW BAR WAS DISCONNECTED. I DID THIS FOR 2 LEGS. RECOMMENDATION: IF; IN FACT; IT IS NOT A REQUIREMENT TO PIN THE NOSE GEAR OVERRIDE PRIOR TO PUSH AS LONG AS THE #1 IS STATED AFTER TOW BAR RELEASE; THEN THE MEL REQUIREMENT FOR OVERRIDE PIN INSERTION PRIOR TO PUSH SHOULD BE REMOVED FROM THE MEL.