Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is the Rammus build I use. It is not meant for damage like some others but it's just meant to be a great tank.
The item selection varies from time to time based on what the enemy team is. I almost always go for Merc Treads for the magic resist and the passive unless their team has little stun/slow/snare/ect.