For an in depth history of the development of the K&E 4093 and 4083 slide rules see my paper, "Mendell Penco Weinbach and the K&E Log Log Vector Slide Rules". This is found on the Oughtred Society's web site. Go to: http://www.oughtred.org. In the Menu on the left click on JOS Plus. When the JOS Plus screen comes up scroll down to March 20, 2008 and click on the Article to read it. The initial source of the K&E slide rules that are listed was Michael O'Leary's, Keuffel and Esser Slide Rule Master Cross Reference (K&E XREF) list that he emailed to me in July, 2002. The main source was Clark McCoy's extensive web site showing dates and scans of many of the K&E Catalog pages. Find this at: www. mccoys-kecatalogs.com. I checked the slide rules in my collection to these two sources, and added the gauge gauge marks to the list. Richard Smith Hughes wrote an Article, On the Evolution of K&E Vector Slide Rules, in JOS, Vol. 14, No.1, Spring 2005. I have made some changes in the list from the comments he gave to me.