ALTDEV ALT OVERSHOT. FLC FLYING AN ACFT THAT DOES NOT HAVE AN ALT CAPTURE MODE GETS DISTRACTED AND CLBS TOO HIGH.

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WE WERE FLYING FROM LAX TO GDL (GUADALAJARA) ON AN IFR FLT PLAN. FILED FL330; ROUTING: LAX-MZB-TIJ; J1 TO SRL; UJ23 TO LMM; UJ3 TO GDL. AFTER TKOF; ZLA CLRED US TO FL290 WITH THE PROVISO THAT WE CROSS TIJ AT OR ABOVE FL280. WE WERE JUST BARELY ABLE TO COMPLY AND AT FL280 OVER TIJ WE WERE HANDED OFF TO MAZATLAN CTR. RATE OF CLB WAS 300-500 FPM CLBING ON THE AUTOPLT. WE CONTACTED MAZATLAN CTR JUST AS WE WERE PASSING TIJ AND TURNING OUTBOUND ON J1. SINCE WE WERE EQUIPPED WITH OMEGA/RNAV; WE REQUESTED TO BE CLRED VIA UT1 TO LMM (A MORE DIRECT RTE). MAZATLAN RESPONDED BY CLRING US TO LMM VIA UT4 WHICH CAUSED US SOME CONFUSION AT THIS POINT SINCE UT4 WAS NOT PRINTED ON OUR CURRENT CHARTS. WE REQUESTED VERIFICATION; AND AFTER 15-20 SECONDS MAZATLAN STATED THAT UT1 HAD BEEN REDESIGNATED UT4. AT THIS TIME MAZATLAN ASKED US TO VERIFY OUR ALT. THE FO RESPONDED THAT WE WERE AT FL290. HE HAD LOOKED AT THE FIRST 2 DIGITS OF THE ALTIMETER. I NOTED THAT WE WERE ACTUALLY AT 29800 FT. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND CORRECTED TO 29000 FT. THE AUTOPLT IN USE WAS 1 OF 2 TYPES INSTALLED ON OUR B-727'S. WE WERE USING THE BLOCK IV; WHICH DOESN'T HAVE AN AUTOMATIC LEVELOFF FEATURE. THE OTHER TYPE; BLOCK V; DOES HAVE AN AUTOMATIC LEVELOFF FEATURE. WE HAD ALLOWED OURSELVES TO BECOME DISTRACTED DURING THE CONFUSION OVER THE ROUTING; AND DUE TO THE NONSTANDARD AUTOPLT LEVELOFF FEATURE PASSED THROUGH THE ASSIGNED ALT. WE LATER DISCOVERED THAT THE CHANGE OF AIRWAY DESIGNATION WAS POSTED IN THE APR/FRI/95 COMMERCIAL CHART NOTAMS; WHICH I HAD SCANNED EARLIER; BUT FAILED TO NOTICE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING A B-727-200 WITH AN AUTOPLT THAT DOES NOT HAVE AN ALT CAPTURE MODE. THIS TYPE OF AUTOPLT REQUIRES THE FLC TO EITHER USE A VERT SPD WHEEL ROLL DOWN TO '0' OR FOR ALT HOLD TO BE ENGAGED. IF THE ALT HOLD TECHNIQUE IS TO BE EMPLOYED THE FLC SHOULD DECREASE THE CLB/DSCNT RATE TO A NOMINAL RATE TO AVOID A SHARP PITCH INPUT WHEN THE ALT HOLD SWITCH IS ENGAGED. THE ARTCC CTLR DID NOT COMMENT CONCERNING THE ACR'S ALT OR ANY TFC CONCERNS.