Here is a thread that talks about what to look for (thread is actually about a Caprice, but further in there is discussion about a 409....keep looking)
https://www.yenko.net/ubbthreads/show...0/fpart/2/vc/1
Verne Frantz is a great resource here. Unfortunately many of the things to look for to authenticate a real 409 can be installed on a clone so as others have said owner history and documentation is helpful. We have a real one so you can PM me if you like for more information. Good luck.
Jake