I feel a lot more upset by makers who "steal" another maker's pattern and produce it as their own, than someone having a blade they legitimately own altered to their specs. (For the record, I've never done it.)
That said, Kit Carson told me when it was obvious to me that another maker was copying his Model 4 in virtually every respect, that "nothing was new under the Sun" and he didn't lose any sleep over it. (Kit was a better man than I'd have been under the same set of circumstances.)
I think both sides make good points here, but as a non-maker I think that once a customer buys the knife, it's theirs to do as they like. That said, if the knife has been modified, it is their responsibility to make sure that folks know that the result is not the original maker's work.
A very interesting and nuanced discussion! Agreed on "nothing new under the sun", but the decorum of this topic nevertheless had a learning curve for me. When I was (more) green in the knife making community I had a notion that "if a design is discontinued, you're not stepping on anyone's toes by making an iteration of it".
And from a purely legal perspective, I think this holds up as long as the design is not being claimed as one's own, but from a courtesy and etiquette perspective, I now believe it is a good deal more nuanced than that, since there is still a question of moral rights, and community norms that I previously failed to consider, to my shame.
Two examples:
Early on I made a Fred Perrin Street Beat profile when a customer requested it, also thinking that because Spyderco had discontinued it, it would be perfectly okay. Made it, posted pics, people liked it, and the customer was pleased. I had requests trickle in for a couple more, and by the second of third one, it clicked with me that I had never even considered whether or not Fred Perrin was still active and making the design himself. I looked him up, found him on Instagram, and reached out. He was kind and said (paraphrase for brevity) "If anyone asks you to make one, go ahead, but don't make them to put up for sale". Made perfect sense to me, and since then I have been resistant to making more of them, even if requested.
Another face palm moment, I was once looking for a CPK knife, and someone offered me a trade, saying basically "Hey CPK discontinued this boot dagger model, can ya make me a knife with this profile and I will trade you this other CPK knife?" At the time I jumped on it because, in my lack of thinking it through, I oversimplified it to "Well it's discontinued, and so I am not competing with CPK"... I don't think I violated any laws, but I made it worse when my steel and blank cutting provider messaged me and said "Hey you still have room on this sheet, and I could fit a bunch more daggers on it if you want", to which I just said, "okay sure". All without reaching out to CPK..... I'd already put half a dozen or so in the wild before it came to my attention that Nathan was put off by what I'd done, and I think my face turned about ten shades of red in that moment, as the floodgates of common sense hit me. Needless to say I didn't make any more.
All that said, the more a knifeās profile is dictated by function the less likely copyright will apply to the shape itself, and it is more than possible for two makers to independently design nearly or even completely identical knife profiles. I saw a knife design by our beloved Crag the Brewer, and I asked him it it was meant to be "Wharnanimous". I admit it was a little jarring to see it at first. But he told me that my design was not even something he had considered when coming up with it, and said that it's just a wharncliffe with a certain blade to handle angle, and pointed out a subtle, but definite difference in the handle shape. I trust and love him, so I accept his answer. He had come up with a knife that suited a specific style of function and ergonomics he wanted to build for, and it just happened to look very similar to a knife I designed to fill very much the same role. I don't take the existence of his knife as a violation of ethics or stepping on my moral rights, or intellectual effort, but as the coincidental overlap of our views on knife design.