Originally Posted by pereb27
The last 2 numbers only determine the region. The letter also determines the HDD size.
2501 = USA, 2504 = Europe. A = 120/160GB, B = 250/320GB.
There is no way to make the difference between downgradeable 25xx and undowngradeable one, apart from looking at the date code at the bottom of the PS3, or using the MinVerChk PUP.
Note that the PS3DevWiki lists 250x and 251x separately, however this is irrelevant, I believe it was said it's because the 160/320gb model was released at a later date in Mexico (region 11) and Hong Kong (region 12). 25xx is 25xx, as I said above, the last two numbers only determine the region.
Thanks for clearing that up. It all makes sense now. Reading the wiki page now and I understand.