Well, actually Leung Shum`s teacher was Ng Wai Nung who learned from CHEN ZI ZHENG - (CHAN TZI CHING).
The connection between the two is from Lau Sing Yau.
Lau Sing Yau passed this knowledge to his third son Lau Kai Man and his nephew Chan Tsi Cheng (Chan Tzi Ching). Lau Kai Man passed the Eagle Claw Kung Fu to his nephew LAU FAT MANG