Identifying my 405??....

[tommy1987]

Hello all,
I bought a 405 rolling shell a year or so ago with no V5C and didn't pay a massive amount of attention to any details when I bought it. It looks like a drophead with all the correct parts but I'm leaning more towards it being a conversion.
I went to look for the chassis tags and there are none on it so I looked up an old thread on here regarding someone's 404 locating the chassis stamp. Ground off some paint on the chassis leg where the 2 holes for the rivets would go and I have the numbers 4001.
Can anyone help in how that translates into what car it is and how to acquire a logbook from here?
Thanks a lot
Tommy