When searching for a part to fit obviously one needs the correct part number but one also has to check effectivity. The part catalogue may list 20 different fuel pumps but will also list effectivity, so a part may be Eff DA all Eff DA 401 - 420 or even DA 356 (only being inferred)
The fuel pump is a good example as it may also be listed as ETOPS or non ETOPS, that would not stop it being fitted but would prevent an ETOPS routing.
Old Boeings say 757/767 can be a nightmare as there are so many variations.
For a JAA operator there is also an issue of certification.
Complex innit
