I was hoping this was the Casino on Squirrel, but I am now convinced that it is the same stage you see below.

This picture is taken from "Souvenir Hand-book of Boothbay - Harbor - and Vicinity" by F.B. Greene, published in 1896. Note the floor lights at the front of the stages and the pattern in the wood at the front of the stages match. Also note that both stages have a "rounded" frame. The caption on the picture actually says "PYTHIAN OPERA ROOM" and the page facing, not shown hear, is of an outside shot of the building and is entitled "PYTHIAN OPERA HOUSE, BOOTHBAY HARBOR." The building is still standing but I have not had a chance to look at the present stage.