IPad mini MD531LL/A and MD528LL/A
The part numbers you refer to are not actual model numbers, but indicate which country the part would be sold through and hence the pricing and warranty country of origin.

MD531LL/A appears to be the original iPad mini (Late 2012) Model: A1432 WiFi only, white, 16 GB with resolution of 1024 x 768 pixels and an 2.4 GHz Apple A5 CPU. The MD528LL/A is the same but in black.