Reference numbers
No.	Literature
1	Century of the Typewriter, Wilfred A. Beeching, publ. by the British Typewriter Museum, 1974, 1990, ISBN 0951679007
2	Joan Sales,"Date your Underwood Typewriter" - Website : http://www.geocities.com/Heartland/Cottage/5405/underwood/serial.htm, reference to Richard Polt
3	Allowance Sheet 1918, made available by Andrew B. Schrader
4	Ernst Martin, Die Schreibmaschine und ihre Entwicklungsgeschichte(The Typewriter and the history of it's developement), publ. by Johannes Meyer Verlag and Basten International, 1920 to 1949 and reprints
5	Darryl Rehr, Antique typewriters, by Schroeder publishing, Los Angeles, 1997, ISBN 0891457577, see the author's homepage in the Links-section
6	Typewriter Age Guide, publ. by Office Machines and Equipment Federation, London, copies contributed by Mr. Bruce Beard, Australia
7	"Liste der Herstellungsdaten gangbarer Schreibmaschinen" Listing of the production dates of commen typewriters), publ. by Wochenschrift für Papier, Otto Hoffmann Verlag, 1941, Berlin, copies contributed by Mr. Paul Roberts, Netherlands
8	Richard Polt, adapted from the book of Paul Lippman, American Typewriters: A Collector's Encyclopedia (1992) and corrected, based on a list publ. in ETCetera #4
9	"Fabriknummernverzeichnis" (Factory number Listing), booklet which could be purchased optionally with book reference No. 4, edition 1941, publ. by Johannes Meyer Verlag, Pappenheim 1941
10	"Fabrikationsdaten von Schreibmaschinen", listing of typewriter serial numbers for dealer's, 1942 and 1935, publ. by Carl Teege Verlag, Hamburg
11	Ames Supply General Catalog from 1949
12	Information contributed by Will Davis, see : http://www.hometown.aol.com/page3.html and entry in the Links-page
13	Liste der Herstellungsdaten deutscher und auslaendischer Schreibmaschinen, 10 Auflage, 1955, publ. by Burghagen Verlag, Hamburg (small book, pre-owned by the Adler Werke vorm. Heinrich Kleyer, Frankfurt)
14	Leonhard Dingwerth, Kleines Lexikon Historischer Schreibmaschinen (Small Lexicon of Historical Typewriters), publ. by Kunstgrafik Dingwerth, Eichendorffstr. 77, D-33415 Verl
15	Office Typewriter Age List No. 26, publ. by Smith-Corona, by courtesy of Mr. Ron Fuller, Los Angeles, USA
16	American Typewriters - A Collector's Encyclopedia by Paul Lippman, 1992, Hoboken