Featherbed frame identification

Hi all.

I own this Norton frame that looks like an early Wideline Featherbed with some sort of custom rear subframe (similar to a slimline) and what looks like sidecar connections. 
I was told that a Vincent HRD engine was in it, and in fact the frame came to me with plates to connect the Vincent engine and right hand secondary setup.
It has a serial number on the left side of the frame, 122 pointing to a Dominator. The vertical number has only 3 digits, probably there were more but there is an additional hole in the lug.
There is another 4 digit serial on the front frame, left side, on the steering column lug.
There is also a California assigned Vin tag.
All the frame tubing joints are brazed, like the Manx. I was told that the production Featherbeds are arc welded.

I am asking for some help to identify this frame. Someone suggested a surplus Manx frame that was customized for sidecar racing with a Vincent engine. But I have no idea.

Thanks