BUT
salesmen with _ good technical knowledge of what they are selling
Im completely confused... In both cases we have more 'particular' meaning.
a secretary with a first-class knowledge of German?
The article a is more often left out than not in this context, but the meaning is the same either way. Abstract non-countable nouns don't typically have an accompanying article, but some occasionally do for stylistic reasons. Here a might be used to emphasize the idea of a certain kind of knowledge ("first-class" in this case).
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