Initially, I concluded that the letters G. R. were the potter's initials, and 1811 was the date of manufacture. Seems reasonable, does it not? But time has taught me otherwise!
One small clue to what happened here is that on each bust the "GR" and "1811" are positioned in the very same spot. In other words, the letters are numbers are integral to the mold itself, rather than something the potter scratched into the clay. The same molds were probably used by assorted potters over several years, and each time a bust impressed "G.R." and "1811" was the result!
So if these busts were not made by GR in 1811, who made them, and when? Truth is, I don't know. I suspect that the MOLDS for the busts were made in the year 1811. Perhaps the maker was a small potter, who then sold the very same molds to other potters. More likely, he was a professional modeler. He may have put his own initials on the molds---or he may have put this detail on the back of the bust as a label of sorts, to reference the order.
The GR 1811 molds were in use, it appears, for a few years, but Neptune was a hot topic in those days, so others too had a go at capturing his form. The third Neptune bust rendition is as below.