I understand that GM Ma learned not only Tai Chi and Pa Kua from him but also the Art of Bending Bones. He taught at a temple in China just across from H.K.
To answer your earlier question, GM Ma learned from Sun Yu Fung at Ching Wu late in SYF’s life. Some time bewteen 1935 to 1945. Even during this time, SYF only taugh the senior disciples and Ma was just starting so he more than likly learned from his older classmate.
I also could not locate any official register list of students in Ching Wu school in Canton. However that does not mean that it did not exist. After WWII and the 1947 revolution, many documents were burned.