According to Chu Chung Man himself, as told by the Chan family in HK today:
Chu Chung Man's first Sifu was Wong Jit Shing, who had learnt from his grandfather, who in turn was a student of Chi Sim himself. From this lineage he learned the "Fa Kuen system", which consisted of three forms: Peng Kuen, Fut Kuen and Mui Fa Baat Gwa. This "system" had supposedly been created by Chi Sim for Wong's grandfather because he was too old to learn another external styles when Chi Sim taught him. So this is a lineage which is unrelated to the Red Boat transmission.
Chu's second Sifu was Dong Ji, whose lineage is:
Chi Sim - Dao Wah Bou - San Gam - Dong Ji. Over the course of this transmission the original Siulam techniques of Chi Sim had been continuously modified and ultimately resulted in the creation of the "Chong Kuen" and 3 different wooden dummy sets (Heaven, Man, Earth versions). Chu Chung Man had learnt the 6.5 pole from someone called Gam Cheung, who in turn had gotten this from his uncle, Gam Ming, a student of Dao Wah Bou.
There is no mention of other teachers of Chu, at least it seems it he didn't mention any to his successor, Chan Wing Yue, it seems.
Did he teach YM anything, is he YM's secret teacher, the true "Leung Bik"... In my opinion, having looked into this quite a bit myself, I'd say most definitely not.