This is an extremely long shot but may be worth a phone call. Eurotune motorcycles in Queanbeyan NSW were Triumph dealers until a few years back. I bought my original Rocket from them in 2005 and ordered it with a chrome oil tank. They replaced the original black one for me. They MIGHT still have the black one???
(Technically, I suppose it's really mine... but I don"t care. Was never given back to me.)
Google Eurotune Motorcycles for contacts. (They are no longer bike dealers but still have some stuff.) Helen or Anders. You can quote me.
Apologies @mr hunt I have not been around much lately.
I have a couple of used later version black ones T1210259 if you want one cheap (Au$50 + post for you, Au$642.00 from Aussie dealers is outrageous! ), and an NOS early Chrome one A9733603.
Pretty sure they changed during 2005 and only very early 2004 -2005 models came with the different sized oil pipe outlets.
Triumph ROCKET III, CLASSIC & ROADSTER | Oil Tank, Chrome supplied next day (UK only). Available at Fowlers Parts, the UK's No.1 Motorcycle Parts Supplier.