This Tibetan dynasty is known as the "Later Ch'in dynasty" (384-417).
"A history of China., [3d ed. rev. and enl.]" by Wolfram Eberhard
Since the Ch'in dynasty (557-589) this has been the great trade of the place, which was then called by its earlier name.
"Encyclopaedia Britannica, 11th Edition, Volume 15, Slice 7" by Various