We have been around this buoy so many times ....
Other than the clearly defined banned items, there is no such thing as an illegal knife - just an illegal reason for having it where you are or an intent to use it for an illegal purpose. So, a butter knife in a box, with butter and bread and cheese in a public park is perfectly legal; a butter knife up the sleeve of your jacket in a bar would probably be seen as illegal.
It doesn't matter if a knife is fixed blade or not, if you have good reason to have it for legal purposes, you have a right to carry it. A non-locking folding knife of 3" or less can be carried without needing a 'reason' but, if you got it out in a pub and threatened someone with it, it is no longer a legal carry as your intent is not legal.
I do not believe there is much ambiguity in the law at all and the Black Belt Lawyer is quite clear about it in my opinion.