We recently upgaded the boards on these with some new features.  Hence the increase in model number from 3.1 to 3.1.1.  To see the difference or to do the upgrade you can see  here that explains this upgrade in more detail.

To upgrade from a Gen 3.0 or 3.1 to a Gen 3.1.1 you will need this board and also the . 

If you are replacing a Gen 3.0 then you will need this board and also the .

If you are replacing a Gen 3.1 board (but do not wish to upgrade as well) then you will just need this board.

BrewZilla Generation 3.1.1 - Main Circuit Board