Here is a link to New Hampshire's penal code on the NH official government website-
http://www.gencourt.state.nh.us/rsa/html/indexes/default.html
And here is a link from that site regarding NH penal code 159:16, which specifically refers to "metal knuckles"-
http://www.gencourt.state.nh.us/rsa/html/xii/159/159-16.htm
You may notice that the law (159:16) does not make any exception for metal knuckles attached to knives. While it appears that it is perfectly legal to possess metal knuckles in NH, the law seems to make it clear that it is illegal to carry such an item. It's up to you if you want to take the risk that a cop or prosecutor might interpret 159:16 according to the strict letter of the law, and determine that "metal knuckles" are illegal with no exception for them being attached to a knife, but I personally wouldn't risk it.
We have a similar law here in California banning the possession of "metal knuckles", and there is no mention in that law regarding metal knuckles attached to knives. I asked my attorney about this once, and he explained to me that the law had no exception for metal knuckles attached to knives, and that I should never expect a prosecutor to make an exception. He explained to me that as long as the "metal knuckles" could be used as a weapon that they are illegal, period, regardless as to whether they are attached to a knife, or referred to as a "paperweight", or hung on a chain around ones neck and called a "necklace", etc. I certainly wouldn't expect a prosecutor in New Hampshire (or anywhere else) to make any exceptions for a person carrying a knife with metal knuckles attached. I don't imagine any prosecutor would look favorably on anyone carrying a knife in public with metal knuckles attached.
Also, when it comes to all the expense and unpleasantness that comes with being arrested, prosecuted, and possibly convicted of a criminal offense, I would advise you to not just take the word of some guy who works in a store, or some guy on an internet forum (including me), when it comes to what is or is not legal.
My advise, read the penal code for yourself. And if after you have read it you aren't confident that you know exactly what is or is not legal, then consult a local criminal defense attorney. Your freedom, and avoiding being arrested, etc, are certainly worth a phone call to a lawyer. Or you can just err on the side of caution and avoid any risk altogether by not carrying a knife with metal knuckles attached.