A lot of research has resulted in a plausible theory of why some vintage Rado watches have an E appended to the name or 5-digit reference number (pre-1973). The E is thought to signify that the watch, whose earliest examples were powered by an AS movement, is a newer one powered by ETA (e.g., Diastar 1 and 1/E). Read more about this in the Rado Forum at equationoftime.com.