There are no known references specific to Botswana meter marks other than an article written by me entitled Botswana Meters – A Brief History, which appeared as a cover article in Forerunners, the Journal of the Philatelic Society of Greater Southern Africa in November 2001, another article entitled Botswana Meter Marks, in the Nova Scotia Stamp Club Newsletter from June 2004, and this website.
The International Postage Meter Stamp Catalog by Joel Hawkins and Richard Stambaugh (2005) contains a listing of Botswana meter marks. The authors of the catalog, while working on a revision sent a note to the exhibitor stating that “due to all the new stamp types [the exhibitor] found [they] are completely redoing the Botswana pages”. They also asked the exhibitor to examine their new pages prior to publication. Many of the marks shown in the original catalog and on the revised pages were contributed from the collection used to create this exhibit. The International Postage Meter Stamp Catalog can now be found on-line and many of the marks in that version of the catalog are also from the collection of the exhibitor. The WikiBook directs people to this website for additional information.
Other information on meter marks in general can be found at the website of the Meter Stamp Society.