I have just purchased two 'Niagara' bowls, one small and one large. From Mark Hills book Sklo I know they were produced by Mstisov or Karlovarske Sklo around 1960.
Each has an acid etched mark on the base - a circle with three vertical lines within the circle.
I have tried to photograph them but they are very feint.
Can anyone help to identify the acid etched marks?