Hi,
A17 stands for the NON-CHONO SOH, because the Breitling calibre 17 is actually a modified ETA 2824-2.
A13 stands for the CHRONO SOH, because the Breitling calibre 13 is a midified ETA 7750.
Serial number coding:
WXXYZZ
W=case material (in your case: A=all stainless steel, B=two tone, K=full gold, and I don't know what is platinum / white gold sorry)
XX= movement (13= Breitling calibre 13, 17=calibre 17, 44=calibre 44 and so on...)
Y= Either a 0 or a 3. 3 means COSC certified, 0 means it's NOT.
ZZ= depends on the model.
Hope this helps.
Greetz.