Mong Kok Tsui (Chinese: 芒角嘴 or 旺角嘴) is the western part of present-day Mong Kok. It is about where Shantung Street and Nelson Street situate.
The name of Mong Kok Tsui was gradually eclipsed by later developed Mong Kok, as reflected by Mong Kok Tsui Market which later renamed the Mong Kok Market.
References
Mong Kok Tsui Wikipedia(Text) CC BY-SA