I know this is an old thread and it has probably been discussed elsewhere too, but if I may, I'd like to add a few words and thoughts.
On the whole I think Sam has summed it up pretty well, and I think there is a fine line between what is acceptable and what not - and I think it is possible to cross over that line.

The question is, where is that line? Another complication is that it is probably not the same for every one...

A few thoughts, which are obviously only my opinion, from which you are welcome to differ, as is your good right.

There are very, very few, if any, realistic knife designs that are new. I am not talking about blades with three tips and more spines than a porcupine - those are just fun/art/show off stuff - I mean working knives.

Sure, you get very individualistic interpretations of those same designs, and some are unique/distinctive - like Randall knives. Or Scagel, Loveless, Ruana, Rexroat etc for that matter - and a good few others. When you see one of those, you just know it is a Randal/Scagel/Loveless etc.
From the perspective of a knife maker, and on condition that it is not a truly unique design, I don't think there is any thing wrong with:
- Making your own interpretation of that knife, loosely based on the original
- Making a tribute knife or knives - either a perfect copy or a very close interpretation of a specific knife and naming it as such. And as a matter of courtesy, if the maker is still around, ask permission. For instance, if I made a custom copy of a Fairnbairn Sykes for a client I would not name it anything but a custom copy of the FS.

What I would find very upsetting is if someone copied a design of mine, made them en masse and marketed as their own, and even using my model numbers/designation, without permission. No matter how well made, no matter what the quality, I'd be very unhappy about it.

Where it does become a very grey area is with some designs - lets take the Randall No 2 Stiletto as example. That I believe falls into the first category I mentioned above - an acceptable own interpretation of another knife - even the Randall shop says it is based on the British (Fairbairn Sykes) fighting knife - although it is larger and substantially more robust. And the FS in turn is based on an age old design - a dagger type that dates probably from at least 1500 to 2000 years ago. No one could claim exclusivity on that knife or probably even an interpretation of it.
But I think this serves to illustrate the last point I made about making copies and using the same model numbers - at the danger of repeating myself, this is an age old design, which has been done in so many different interpretations, that no one could reliably claim exclusivity on it - it is a dagger/stiletto and that is it.

So, to me, if one was to make a copy and designate it as the "No 2 Fighting Stiletto" on a knife that was a dead ringer for a Randall, even to the point of the same style stamping, it means only one thing - you are trying to take a ride on the back of hard work done by others....

I like the Randall Knives. A lot. It would probably be a long time before I could afford another one. In the meantime I may well make myself a copy of the #25 for instance. But I will always call it that - a copy/tribute knife and for my own use only. If I wanted to make a similar type knife for production, I might well use a Randall, or a Scagel or some other knife as "inspiration" but I would make sure that it is my own interpretation of that knife, give it my own model number and still give credit for the inspiration - much like Mr. Bo Randall did with the Scagel knives.

My 2c