I'm taking inspo from all over this joint and I remember some months ago I saw a bike that had a rusted tank on one side.. It was a bit of a scrambler I think, but kept the same seat...
I know that's not much to go on but does anyone remember the thread?
If this is the bike you are referring to.....
(https://farm2.staticflickr.com/1844/42789827650_337aa7104b_c.jpg)
Then it's from iamhiding in this thread.
http://gstwins.com/gsboard/index.php?topic=69916.0