The following four pics are all "Triangle M" porcelain signals (nickname for Illinois Elect. Porc. Co. during their production years where they marked insulators with a "M" inside of a triangle). The have been ink stamped on the dome (I believe they used dark blue and/or black ink) but this marking is lost when the insulator is glazed dark brown. The ink stamp occassionally makes a slight indentation which helps in spotting the mark, but for the most part the marking is essentially invisible. Photograph under fairly bright white lighting, turn the pic into a negative then tweak the contrast/brightness and sometimes the color saturation and one out of about every five no name Triangle M's will actually have a marking.