The G stands for gold. The reading one occasionally hears, that it stands for glatt, the German for smooth, and refers to a smooth surface on the cone, is wrong. The letter thus describes a trim variant of the model in question and not a series of its own; a 146 G is a 146.
Not every example carries this addition in its imprint, which is why the absence of the G on its own should not be taken to indicate a different version. Imprints on the cone and the piston knob are, moreover, often rubbed away by decades of use and barely legible in unfavourable light; viewed under raking light they will usually reappear. Anyone wanting to identify an inherited piece with confidence should therefore draw on further features, above all the hallmark on the nib, which gives the gold content as 585 for 14 carat or 750 for 18 carat, and the shape and number of the cap rings.