Help Needed Identifying Triumph Motorbike

http://classic-motorbikes.net/images/gallery/triumph_500_nt_small.jpg

It might be a variant of this, type 494cc triumph NT on googie and the research can begin, sorry don't have time to bottom it myself, hope this provides a start. A frame number would help a lot to find exact year. I suspect a bit later looking at forks and foot change? Someone could have fitted different engine so frame number VERY important.

Article says
"Very rare motorcycle indeed, a triumph 500cc nt from 1930 or jan '31, at the time it was made, the nt was the fastest bike in the triumph range, having a twin port over head valve engine 494cc and 3 speed hand change."

 

Here you go

 

Yeah he is in for a lot of work finding info, I think Triumph owners Club have an historian as a lot of Clubs do. Velocette Historian helped me greatly regarding my Dads 1957 bike. He should be able to answer/direct. Good luck.

 

Any progress here?
I also own a triumph with the same specifications, my bike should be a model CD.
I found an old picture.

Anyone with more information about what year it was build.
I also found this

 

Some more pictures I found