I'm no guru...just some lamen who notices silly things.
On the rear door, where the door would normally have a gap, there is an evident rust stain running vertically exactly where you would expect to see a door gap.
This could be where the original door was welded up and the rust is coming through
?
Also if you follow the lower edge of the door, seems to be what looks like bubble flaky bog
Just an observation of a photo...I'd say a one of a kind custom or home job.
Either way.....a great find ! Is it worth grabbing, or is it really far gone ?
Mark