Hello Margot King,
on first sight, there is no place given today named "Hessum". A similar sounding place in Lower Saxony today is "Hessens", which is a part of "26384 Wilhelmshaven". There was given in 1998 a "HOLTMANN, I., Ulmenstr. 29, 26384 Wilhelmshaven, Tel. 04421 36386".
Holtmann is no rare name, Kalkmann is it much more. The spread of both names is not centered in Lower-Saxony, but in the common region with North-Rhine-Westfalia. For example you find both names given to a certain amount in the town "Osnabrück". Osnabrück belongs to Lower-Saxony since 1823. So it might be perhaps wise to widen your research on this common region nearby the border to the Netherlands as also the spread of Holtmann is much more in the south-western parts of Lower saxony than in the North and East of former Kingdom of Hannover.
Perhaps someone else knows better and more advice.
Good luck in your research
Hans Peter Albers, Bienenbüttel
"Margot King" <firstname.lastname@example.org> schrieb: