The Manhard imprint published just under 100 titles from 1973-1979.
The style of the cover art would only change once near the end of the imprint's life. All the black ink line drawings appear to have been done by a single artist, or a pair of artists with similar styles, whose name(s) remain(s) unknown. Sadly, there are also a number of covers I have been unable to locate.
Here are another dozen (or so) titles: