A B757-200 ACFT MECHANIC INTERPRETED THE STAMPED DATES ON THE APU AND ENGINE FIRE BOTTLE SQUIBS AS THE EXPIRATION DATES.

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JUNE OF THIS YR; ACFT WAS IN A PHASE CHK AT ZZZ. I WAS TASKED WITH CHKING THE APU #1 AND #2 ENG FIRE BOTTLE SQUIBS FOR EXPIRATION ON THEIR 10 YR LIFE CYCLE. I WAS NOT GIVEN OJT TRAINING BEFORE PERFORMING THE TASK. I INTERPED PARTS OF THE STAMPED NUMBERS ON THE SHOULDER OF THE SQUIBS TO BE DATES. THIS ACFT WENT TO HVY CHK AND IT WAS FOUND THAT THESE SQUIBS WERE NEAR EXPIRED OR EXPIRED. THIS ALSO HAPPENED TO THE ACFT THAT FOLLOWED ACFT X INTO HVY CHK. AFTER RECEIVING OJT IN REF TO THE ACFT X OCCURRENCE; I REALIZED THE EXPIRATION DATES WERE ETCHED AND NOT STAMPED ON THE SHOULDER OF THE SQUIBS. I SUGGESTED AND MY COMPANY WILL MODIFY THEIR PHASE TASK CARDS TO REQUIRE A DATE AND SERIAL NUMBER BLOCK BE ADDED TO THE TASK CARDS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE LASER ETCHED NUMBERS ON THE SHOULDERS OF THE APU AND ENGINE FIRE BOTTLES ARE THE NUMBERS TO DETERMINE THE EXPIRATION DATES; NOT THE STAMPED NUMBERS. HIS COMPANY IS MODIFYING THEIR WORK CARDS TO INCLUDE A SECTION TO HAVE THE ETCHED NUMBERS RECORDED ON THE INSPECTOR AND MECHANIC'S PAPERWORK.