In a downloaded set of vocabulary flashcards, I found "Unterrichtsbuch" given as the translation for "textbook". Online English-German dictionaries seem to prefer "Lehrbuch" for this, and in fact "Unterrichtsbuch" seems very uncommon: the only reference work I've found it in is the old Jacob and Wilhelm Grimm dictionary. However, I do find it in online texts and corpora, for example in this 2015 Spiegel article.
Is there any difference in meaning between these two terms, or can I regard "Unterrichtsbuch" as an exact but uncommon synonym for "Lehrbuch"?