is there a date for the recipe or is there a LIGHT or DARK treacle specified in the recipe? because I think the term "treacle" was used differently back in the 19th century, it referred to refined, pale molasses, but today it's used to refer to molasses, with light and dark treacle referring to light and dark molasses.
Randy Mosher in Radical Brewing describes light molasses flavor as full, rummy with medium heaviness. dark molasses as very full, a little tarry and blackstrap as heavy, tarry; may be astringent