Let me give you my take on it. (This in no way is meant to be legal advice, or to induce you to act or refrain from acting in accordance with my opinion. It also does not create an attorney client relationship.........there.....).
If you jurisdiction has any law regarding metal knuckles (as mine does here in Washington State), I will suggest that an officer who has a problem with you, will not care that the metal knuckles also have a sharp (often double edged) blade attached, and then decide not to charge you based on that. On the contrary, that will likely make him more likely to charge you.
In my jurisdiction, metal knuckles are considered "contraband" meaning it does not matter if you are carrying them out in the open, or concealed. Just the possession of them is a crime.
I would then suggest, that addition of another dangerous option, making them instantly lethal is not likely to sway the officer into buying the argument that they are not "metal Knuckles" anymore because they are a knife with metal knuckles.
I have had quite a few cases where metal knuckles resulted in charges.
That said, I have not had a case of a trench knife yet. If carried in the open, it might not result in charges, depending on time/situation/behavior. Google for your local statutes.
There is a reason that lawyers won't give a yes or no answer to a lot of questions........we rather prefer "depends".
I have had clients charged, and hauled into court many times on charges for conduct that was not illegal. Once pointed out, the prosecutor some times says "set it for trial". The better ones will dismiss without going that far, but usually you have been charged, missed work about 4 times, and incurred cost and headache before that stage is met.
Just thinking out loud here.
The local officers feel that spring assisted knives are, in fact switch blades or "spring knives" because they have spring assisted in the title, and "look your Honor........they sure do open fast.......Just like a switch blade"
I have, I believe, convinced my local prosecutors, that they are legal to possess, and carry, in part, because I was able to point out the Federal statute specifically exempting them from classification as a switch blade, and exempting them from the export ban on such.
But the Howdy-Dooty officers will keep on arresting people on it, I guarantee you.