Nogo and Singo Origins

[David]

I've been searching the archives looking to see if this has been discussed, but have not found a thread yet.
When did the Nogo and Singo forms of keris develop. Did these come about in the Islamic era. I am not sure i have ever seen either of these forms on a keris from the Mojopahit period. If anyone has images of example of Mojo era nogo or singo keris could they please post them. Thanks!