Prior to the early 2000s, "duplicate" 337s were widely used. If one like aircraft had a major repair/alteration approved you could extend that approval to other like aircraft administratively through the GADO/FSDO. After about 2002, the "duplicate" 337 went to the wayside and each aircraft required it's own separate field approval. It was also the time the issues of "major change" and STCs came about with certain aircraft alterations. Can you still use a previously approved 337 as part of your data package for a major repair/alteration? Sure. But it is no longer an automatic approval. Your A&P/IA can give you more specific info to your aircraft.