Please name those authoritative sources, Don! I'm always happy to be proved wrong, but I do want evidence; even a single example of the use of the term "weapon knife" in a ballad would be a start; unless we're expected to believe (by people who weren't there, it seems, but still know better) that all those "wee pen knives" were mis-hearings of what the singers actually sang. Until then, I'm going to remain stubbornly convinced that the suggestion is nonsense.

See last November's thread  Wee pen knife. You didn't post to it at the time.