The best-known medieval Chinese name for Tibet is Tubo (Chinese: 吐蕃 also written as 土蕃 or 土番; pinyin: Tǔbō or Tǔfān). This name first appears in Chinese characters as 土番 in the 7th century (Li Tai) and as 吐蕃 in the 10th-century (Old Book of Tang describing 608–609 emissaries from Tibetan King Namri Songtsen to Emperor Yang of Sui). In the Middle Chinese spoken during that period, as reconstructed by William H. Baxter, 土番 was pronounced thux-phjon and 吐蕃 was pronounced thux-pjon (with the x representing tone).

Answer this question, if possible (if impossible, reply "unanswerable"): When did the name for Tibet, Tubo, first appear in Chinese characters?
7th century