Unknown BSA trials tank

[baldeagle]

I have just bought a BSA B40 with unknown fibreglass petrol tank. It has the bolt hole in the middle to fit the BSA but it's an unusual tank so I'd like to find out what it is.
One suggestion has been a tank that Fluff Brown/AJS was using for their Cotton scrambler re-issues a few years back. I can find no pictures of these bikes, any Cottons I do find do look quite different.
It does appear too good to be home made.
Better pics can be seen at

http://groups.msn.com/BritishClassicMotorcycles/general.msnw?action=get_message&mview=0&ID_Message=252&LastModified=4675612872195054407

Thanks for looking.