Gavin:
For what it is worth, here are some links to online pictures of so-called Naga knives in a local museum. The University of Pennsylvania is a prestigious university with a long history of anthropologic research. However, I'm not sure that it is a great authority on Naga artifacts.
These are the Naga knives listed in the online catalog of the University of Pennsylvania’s Museum of Archeology and Anthropology:
http://www.penn.museum/collections/object/23189
http://www.penn.museum/collections/object/58188
http://www.penn.museum/collections/object/203252
http://www.penn.museum/collections/object/246272
http://www.penn.museum/collections/object/80616
Ian