Nobody Important,
There are stories floating around out there that state Ng Chung So passed on some White Crane, one source you can look into for that is with Chan Yiu Minās line. Depending on the style and or lineage, this form has several names ranging from Che Chin Kuen, Jin Kuen, Chuan Sin Jeung, Seung Kuang Chong, Chong Kuen etc. Iāve already stated that I believe that this was a set of San Sik passed on from Leung Jan. There is evidence for this in the movements as they correlate well with the known San Sik set passed on by Leung Jan, namely La Jin Choi, Lin Wan Kau Da, Pien San Choi etc. The unfortunate thing about Ng Chung So, is that most of what he taught and whom he taught were eradicated during the Cultural Revolution. We have very few known sources that can rebuild a picture of what this man passed on. This is why I said he wasnāt really well known, in his day perhaps he was, but today his legacy has been all but wiped from this earth.
The ChaN Yu Min lineage in Seundak passes on a lot of things which are very much at odds with the Wing Chun Pai in Fatsaan.
Are you familiar with the CYM lineage tree and history?
From the perspective of Fatsaan Wing Chun only about 25 percent of what the Chan Yu Min people do is Wing Chun.
Legends and stories aside - and the fact that Seundak Siulam Weng Chun is excellent Gong Fu, IMHO - if we make a technical comparison between Fatsaan Wing Chun and Seundak Weng Chun, it is clear that there are some blatant and massive differences.
You mentioned a form called Sei Muhn - and I said that the only form having that name in CYM style is the "Siu Lim Tao Sei Muhn".
For the people who don't know what it looks like:
People can make up their own minds what is what...
Looking at the second part - the Sei Muhn - it doesn't really look like anything found in the Che Tsin Kuen form by Fohk Chiu.
As far as the other forms you mention (Lin Wan Kau Da, La Jin Kuen, Pin San Choi), unless they have other "official" names, there are no such forms amongst the 12 in the curriculum of Seundak Weng Chun - at least according to what information is publically available in China. But given that Chan Gok Gei sifu said there was a lot of misinformation out there, I will ask him next time I have the chance.
Anyway, if these forms are like the Sei Muhn part of the Seundak Weng Chun equivalent of SLT it is rather dubious to call these "known Sansik sets passed on by Leung Jan".
I presume you have seen these forms since you said they look like sections of your Cheung Sum Jeung?
Now, this is confusing... You said, your Taisigong came with the form based on some San Sik he had learnt and now you are talking about some form form supposedly from Leung Jan which "has several names ranging from Che Chin Kuen, Jin Kuen, Chuan Sin Jeung, Seung Kuang Chong, Chong Kuen etc."
So which schools exactly have those forms you just mentioned? It sounds like you are talking about Sheung Gung Kuen and the Chong Kuen of Tang family Weng Chun. If so, your conclusion about these being some San Sik forms from Leung Jan be proven wrong for the following reasons:
1. Sheung Gung Kuen is from Yeung Tim, no relation or connection to Leung Jan
2. Chong Kuen was created by Fung Siu Ching because his students wondered why just taught them a dummy form... It is essentially an empty dummy form with lots of footwork added. From the information available, it can be extrapolated that this Chong Kuen was created ca. 1890 which is after Leung Jan taught his sons and Chan Wah Shun in Fatsaan, and around the time he moved to Dongbin, Gulao Seui Heung.
But maybe you are referring to other forms?
If not, how can all these different forms from different unrelated sources be different expressions of one form passed on by Leung Jan?
What is this Pin Sun San Sik you mention, and where is it from?