Using of the indefinite article
There are two sentences with almost the same meaning:
"Steve and Brett both have a first class knowledge of the product..."
"...she is incredibly knowledgeable of her industry, with first class knowledge of her products..."
If the meanings of the sentences are not understandable enough, then I can place the hole ones. Just let me know, please.

Why the indefinite article was used with words "first class knowledge' on the first sentence and was not on the second one?

Is there some influence of the forms of the word "product" here? Why was the article used with an uncountable noun "knowledge"?

Question

Using of the indefinite article
There are two sentences with almost the same meaning:
"Steve and Brett both have a first class knowledge of the product..."
"...she is incredibly knowledgeable of her industry, with first class knowledge of her products..."
If the meanings of the sentences are not understandable enough, then I can place the hole ones. Just let me know, please.

Why the indefinite article was used with words "first class knowledge' on the first sentence and was not on the second one?

Is there some influence of the forms of the word "product" here? Why was the article used with an uncountable noun "knowledge"?

Answer

The rule is: knowledge is in general an uncountable noun, but if you wish to insert an adjective to describe it, you need to use the formula "a [adjective] knowledge of..."

Answer

In these two cases the meaning is virtually the same.  In fact, when I was reading the first sentence I thought that the use of the indefinite article was a bit odd sounding (albeit correct, none the less).  I am sure that there are cases where the use of the indefinite article will create nuanced differences but I cannot think of them off the top of my head and the case that you presented is not one of them.

Good luck!

Answer

The second sentence is incorrect.

"knowledgeable about..."

"with a first-class knowledge of..."

It would be better style anyway to write instead, "She is incredibly knowledgeable about the industry.  Her product knowledge is first class."