I found this information on Margaret MacArthur. There is also a video on the page. The title of her How to Play the MacArthur Harp book is a link to a library website. If all else fails you could possibly find a copy in a library through their search function.

Folksong Collector and Singer Margaret MacArthur

Watching the video - she only plays a very short piece on the harp at the end - I can see what a clever idea it is to have the bass strings at a cross angle to the higher strings. Very neat!

Helen, whose harps are bigger and heavier than the MacArthur harp. LOL