Series E Modified was developed from Series E by thickening the stroke width to accommodate button reflectors for ground-mounted signs, while a lowercase alphabet was developed to allow mixed-case legend (consisting initially of Series D and lowercase letters) to be used on externally illuminated overhead signs.

The lowercase letters, paired with Series E Modified, later became the basis of a national standard for mixed-case legend on freeway guide signs with the 1958 publication of the American Association of State Highway and Transportation Officials (AASHTO)