How do I verify that an Auburn is authentic?

Don't think for a minute that the Boat tail rear is not worth anything.
I'll bet that the"back end" was originally a Sedan.

Thank you for the reply and explanation. It is too bad they replaced the the back end of the car with fiberglass.

I think that you will find that most people on the forum would interpret 'authentic' to mean as equipped from the factory, as in would pass ACD club certification. For Auburns, I believe that would require it to be equipped with the same body number, style and construction as it left the factory.
Past that , everyone's version of 'authentic' will differ. If the rest of the car is untouched and has the original drivetrain and sheet metal then it is likely 'authentic looking'
As has been pointed out on other old car forums , VIN's are actually a specific term for standardised vehicle identification brought in some time in the 60's I think. Not to be confused with prior forms of serial numbers for which each manufacturer, including Auburn, had their own format.
The Auburn engine number is not the serial number.
The data plate on the car ,which should be under the carpet on the right sill, in front of the seat, should have the engine number, serial number and model number.
The serial number will decode to the frame number and begins with the model number and ends with a letter that denotes the body style.
You already know that the current body style will not match the serial number ( unless someone replaced an original metal over wood boattail body with fiberglass) so the authenticity pretty much stops there.
Hope that helps. Not trying to knock your potential purchase. It sounds like a fun car and I suspect there are a lot of fibreglass boattail owners out there who are members of the club , are welcome at events and enjoy their cars but I don't think they would be accepted as authentic. I'm not sure that even the wood frame and sheet metal boattail reproductions, which would be considerably more 'authentic ', would even be considered so.
Brad

I don't think I made myself clear. The ONLY part that is fiberglass is the boattail. The rest of the body and fenders are metal. The engine number is GU72931 which I understand is the VIN,

If it is a fiberglass body it is not original period. R

Please contact the club Auburn historian and share the serial numbers with him. From what you are telling this forum this may not be an "original" car as it came from the factory. They did not make fiberglass body parts back then.

I am considering buying a 1931 Auburn Speedster that had frame off restoration. It looks really nice. The owner claims the car is original including the engine, drive train, and interior. The only item the owner says is not original is the boat-tail section which was fabricated with fiberglass. Is there any way I can verify this is not a reproduction and is in fact a 1931 Auburn?

Thanks for your help.