Which pulley is which on my Shimano STX RC derailleur?
From your photo, the left pulley (beefier, with metal bushing) is the top one, which goes closer to the cassette.

I know that because that's how things were in every shimano derailer I had over the years, and I think that makes pretty much sense, since the beefier pulley (the upper one) actually shifts the gears, while the bottom, thinner one is only an idler that keeps the chain tensioned.

As for the "centeron G" stuff, I have no idea what this means...