I have always attributed the SG rifles to Samuel Glover, mainly because of an old collector in Sullivan County, TN, by the name of James Moran. Jim was quite a Glover fan. He said Glover lived just down the road from Alfred Farrington Gross. I don't know how close together they lived because the census of 1860 puts them in different civil districts. Glover was born in Sullivan County in February 1828 and worked in Sullivan County near Bluff City. Gross was born in 1823 and worked near Piney Flalt.
I don't know for sure what the connection was between these two men but I think Glover spent a great deal of time working with Alfred, or maybe they both worked for Jacob Gross. But Glover's work is very similiar to Alfred's. There are certain details that they both did that are nearly identical.
I have owned several SG rifles and have seen several more. I think your rifle is the best I have seen. The cigar box is a better design than most and this is the only rifle I have seen with an iron trigger guard. The others have all been brass and I don't recall seeing two alike. They usually have a great iron butt plate and that cast brass trigger guard. I would be curious to know if the front end of the trigger plate has a straight depression about a quarter of an inch wide and an extension on the trigger guard bow that fits into that cut out, another Alfred Gross detail. The tang on this gun is about typical for Glover. Gross sometimes used a tang that went all the way down under the butt plate.
Because of the points I've mentioned and the shape of the stock, I think this is one of Glover's earlier guns.
Louie Parker