"Prime" is a word used to describe OmniMechs - a particular subclass of BattleMechs.
BattleMechs are "fixed" configuration - so a Marauder MAD-3R has one set of weapons in particular placement, a MAD-9S has a different set of weapons on a similar chassis, but you'd need a major factory-style refit, typically, to change from one config to another.
OmniMechs are different, and in most ways better. The chassis and armour is set, but weapons & equipment can be changed around relatively easily - "plug and play", almost, given you've bought the weapons & equipment in appropriate pod-mounts. The Clans developed Omni tech first, but in later years the IS embraced it with various degrees of enthusiasm.
So let's start with the Mad Dog. It's a 60-ton OmniMech. Typically it loads up with a large and medium ER laser in each arm, and an LRM-20 with a ton of ammo in each torso. This particular configuration is the most common, and is labelled the "Prime". There are several other commonly occurring configurations available, replacing the weapons mentioned before with varying combinations of other good things. These are typically labelled the A, B, C, etc configs.
There are a number of moderately consistent 'odd' configs, mainly for Clan 'Mechs. H configs often carry Heavy Lasers. U configs are optimised for under water work. S configs suit urban terrain. But not always.
(Technically there's no reason why any Omni should have 'set' configs. It's done mainly for game balance, and to help opponents have some idea what a given Omni is carrying.)
Now the Centurion is slightly more complex (of course!) The Centurion BattleMech was developed in 2801 game time, and for three hundred years existed in a number of factory-built 'fixed' variants. In 3120 or thereabouts, a game faction too the time to develop an OmniMech based on the Centurion, which they called the Centurion. Players will typically call this new design a Centurion Omni. For reasons too long to go into here, the Centurion Omni looks quite different from the old Centurion - mainly due to artistic aesthetics changing between the 1980s and 2000s, rather than for any in-game reason ;)
So the Centurion Prime you like is the mini for the 3120s-era OmniMech, in its Prime configuration - cannon, missile launcher, and two lasers. Several standard omni configurations exist - some of which look similar to the Prime, some of which look very different (like the D config with axe and shield).
In game terms, it's not really an issue. BattleTech is not, and has never been, a WYSIWIG game. So you can quite happily use your Centurion OmniMech mini in play as a Prime config, a D config, or even as the older Centurion BattleMech, as long as you make sure your opponent knows what you're doing. If we can cope with bottlecaps, coins, and small statues of the Buddha being used to represent units - "proxying" - then this is a no-brainer O0
Given the Centurion OmniMech's Prime config is pretty much equivalent to the older version's, I'd say go wild!
W.
