I think you have a few things mixed up.
Let's start with んです. This is not just one thing. It's two: ん＋です where ん is just short for the nominalizer の. Generally, this may be untranslatable when you try to bring it back into English. But, in Japanese it's serving to give some kind of explanation for why you did something, think something, or why something happened. As a nominalizer, nouns, adjectives, etc have to be properly inflected before it.
If you wanted to say, "It's a dog", then you could say
That would be formal. Informally, it would be:
But let's assume that someone questioned you about something and you're trying to explain yourself. Then you would possibly use the んです construction and say
It's a dog.
Here な is the form that the copula だ takes before a noun it's modifying.
When you're saying something like
This breaks into three parts:
そうな ＋ ん ＋ です
where そうな is the form that そうだ takes as a relative clause.
Put another way
そうなんです ＝ そうなのです
A more literal translation of this would be
It's the case that it is thus.
そうんです is just ungrammatical.
なんです and んです are not at all the same thing. Neither are really complete sentences. The first parses as
な ＋ ん ＋ です
[copula] + nominalizer + copula(formal)
The second parses just as
nominalizer + copula(formal)
which doesn't make a whole lot of sense on it's own like that.
Now consider your example
This is really a sentence followed by the nominalizer の acting as providing an informal reason for something about to be said or already discussed.
The sentence is
To nominalize it you can think of it going through the following steps
この包丁は有名な刀匠が作ったものです ＋ の ＋ です
change the first formal です to its informal form だ
この包丁は有名な刀匠が作ったものだ ＋ の ＋ です
change だ to the form it must take in a relative clause, な
この包丁は有名な刀匠が作ったものな ＋ の ＋ です
contract の to ん
この包丁は有名な刀匠が作ったものな ＋ ん ＋ です