SKUs and Build Numbers

Hey everyone,

Could someone clarify the SKUs and Build numbers for me? How do they work?

So, I bought my G4+ on Amazon India.
SKU - XT1643
Build - MPJ24.139-23.1

Now, I searched through the FileFactory dir , but I'm unable to find the stock images for this combo.

I can find XT1640/1 with build MPJ24.139-23.1, or XT1643 with build MPJ24.139-13.3, but not what I have by default.
Do I have some rare combo, for some reason?

Thanks! :D