I was looking again at the Finn 1989 patent 4,821,356 for the M9, which can be found here in this website
http://www.m9bayonet.com/library/04821356.pdf
and I found some answers to my questions.
Not really which one is better between a full tang and an a separate one, but still a good description on how the system was supposed to work and some great materials for speculations
.
I don't understand anything on all these materials, but looking at the pictures and description it seems that the key is how the handle is made, and the fact that the tang "
resiliency is greater than that on the blade".
I first thought this would not be achieved with a blade and tang made of the very same material, like in the full tang configuration
Seems I am wrong.
This is what the Buck Knives 1997 patent 5,594,967 says on the topic
http://www.m9bayonet.com/library/05594967.pdf