I'm not sure you'll ever get a definitive answer to this. Most early boats had 4 ports, but in the mid-70s boats appeared with either 2 ports or 4 ports. It could be that the forecabin ports became an extra-cost option. I bought a new Centaur in 76 and paid extra for opening forecabin ports (but I cant remember whether the standard spec was fixed ports or no ports). You can certainly see Centaurs with 2 ports instead of the usual 4.
Mine has 4, but they're badly designed in that they're angled downward/inward if you know what I mean, so water sits it the hole and pours in when they're opened. Because of that I wondered if the foward cabin pair had been retrofitted.