That is correct. The older Solitaire versions were made in 925 sterling silver, with gold plating on top of that in the vermeil version. Comparable models in current production are of gold-plated brass instead. The difference is easy to see on a used piece that has not been freshly polished: silver tarnishes over time, first in the recesses of the guilloche and at the joins with the rings, where slight discolouration appears. A soft cloth will take it off again.
For the buying decision this comes down chiefly to weighing the value of the material against the upkeep. The older silver pieces have to be polished up regularly; the newer brass versions are easier to look after but hold no precious metal. Tarnishing, incidentally, affects not only silver but also the gold-plated surfaces of older series such as Nikolai and Ramses. A further point in favour of older pieces is the surfaces themselves: certain patterns, the barley guilloche among them, are no longer made and can be had only second-hand.