Can that used as an antecedent in attributive clause?

[Matthewxua]

I am an Enlish teacher in China. Recently I ran across Daudet's the Last Lesson(adapted) in our text book in which there is a sentence that puzzled me a lot. Not till then, when I had got a little over my fright, did I see that our teacher had on his beautiful green coat, that he never wore except on inspection and prize days. Personally and grammatically, I think the that I marked in red should be which . This sentece is a nonrestrictive attributive clause because there is a comma following coat. Or there is a special grammar that I don't know, say, that is used as an antecedent in attributive clause and the relative pronoun that is ommited. Could anybody help me please?

[Anglika]

I need to ask - what date was this translation of Daudet? Does your text book give this information?

In modern writing it is a wrong usage, but I have met it in writings of the 19th century.

[Matthewxua]

It doesn't offer any information about date of the English version. I guess maybe the author didn't risk violating the intelligence property of the original one so he choosed to retain it.