ariel wng frame number

[skr]

Hello all,
We have what we believe to be a ariel wng 350 but we cannot find the frame number as it has been stored in a barn for the last 40 years and is covered in surface rust and filth. Does any one know where we should be looking on the frame. I have checked the headstock area but cannot find it. Its aprox 41-45 year rigid frame.
Thanks Steve.