Question: Nsibidi Script

From the relatively limited notices I can gather from the Internet, Nsibidi script seems to be an wholly indigenous Subsaharan African ideographic script (its usage being centerd in southeastern Nigeria and thereabouts), or an extremely refined semasiographic system that come very close to ideographic/logographic writing. I gather that studying it is complicated by the very close relationship it keeps with West African secret societies.
I am puzzled about it as it appears that all detailed academic references about it are either very old or very vague (or both), and most are beyond paywalls anyway.
I find this puzzling because, hey, an independent invention of writing in Subsaharan Africa should be a very important piece of historical knowledge by all accounts, somethings that should be inserted in our general understanding of how historical development of writing systems operates. And this seems not to be the case, which is strange to say the least (I can see the historical root of such biases, but what the hell?).
So I call to the collective wisdom of this board: what is actually known about this script? Can its origins be dated/traced? Are there any serious scholarly sources that are not from a century ago?

Thanks for any suggestion.
 
A Google Scholar search gives citations to articles more recent than the early 20th century, but unfortunately not to the articles themselves. If there's a kindhearted soul out there with access to a university library, I'd like to see them too.

I found this. There appears to be a book as well, but I don't how accessible it may be. I am attending a conference of Africanists in my university tomorrow, so maybe I'll find somene competent to ask.
Nsibidi seems to be something that should be at the forefront of any systematic research on writing at large, both because of its geographical range and it typological characteristics, which are fairly interesting.
 
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